Sure you can tell the players that it is a runic circle. But the players can't then turn right around and have it where their characters just know that it is a runic circle. That's metagaming. Something that ought to be avoided in a role-playing session.
There is no meta-gaming.
Consider: The GM can tell the players, "As you walk outside, you see the sun bright in the sky. It's a lovely day!" It's not metagaming for the players to act on that information - eg to choose to leave their rain cloaks behind, so as to avoid encumbrance from them.
Likewise, if the fiction is apposite - eg a character educated in runic matters looking at a runic circle - then the GM can tell the player of that character, "You see a runic circle. It looks to you like a symbol of protection." Or even, "You see a runic circle. You recognise it as a runic circle."
When it might be appropriate for the GM to say such a thing depends upon various considerations, including some that vary from system to system. I take up some of these matters below.
Runic circles certainly could be something straight out of folklore within the setting.
Why not?
a player can imagine what a runic circle could look like based on whatever you description you happen to give them, and from whatever the genre says about them. And if they are really lucky, they might have seen pics of one in one of their role-playing books. But unless their characters happen to be adventuring in a high magic setting where runic circles might be a daily thing they come across, the sight of a runic circle on the floor is going to be something of a mystery because they don't know what the players know. They shouldn't know what it is right away. The characters will need to work at it via skill checks and by talking about it amongst themselves. Once they do that, then they could confirm it with you.
In my Torchbearer 2e game, two of the PCs started with significant Lore Master skill. In my Burning Wheel game, it's pretty common for wizardly PCs to start with Aura Reading and/or Symbology. In 4e D&D it is mandatory for wizards to start with Arcana skill.
There is no reason to think that a runic circle would be especially mysterious to these PCs.
In each of these games, a runic circle is something the GM might introduce into a scene, and mention to the players. The principles by which the GM might then dispense information about it is varied. Suppose a player asks "What do the runes say?" the GM can simply reply (in 4e D&D and BW, this is "saying 'yes'"; in Torchbearer this would be rewarding a "good idea", of reading the runes). Or can call for a check, if something is at stake in whether or not the PC recognises the symbols.
But this is all about game play. There is no premise that PCs educated in the magical arts will, in general, be ignorant of runes.
And what a skill check represents can vary across and within these games. BW can be particularly subtle in this respect, because of the way its rules for combining skills into a single check work. There is no reason
in general to think that a check represents a character acquiring new knowledge, or cudgelling their wits. On the other hand, in 4e D&D an Arcana check may well represent the acquiring of new knowledge (by "identifying" the magic), and the same is true of Aura Reading in BW.
You don't find it engaging when the players have to pretend to be their characters and not know what a runic circle is. The players otoh might find it engaging to figure out the mystery all on their own.
Whether or not there is a mystery here is entirely in the control of the GM. There are any number of other possible mysteries in the neighbourhood, too, such as - Who inscribed the runic circle? Why here? What were they seeking protection from? Or what were they seeking to protect? Etc, etc.
There is nothing inherently
more exciting about learning the nature of the circle than learning any of these other things, and for my part I incline to agree with
@hawkeyefan that learning the nature of the circle is towards the bottom in this respect, because in itself it does not tend to propel play forward as these other things might tend to.
The knowledge about the runic circle is something that the DM should only know at first. They are the ones who have spent the time and effort reading the pre-made adventure before they have the players role-play it in-game as their characters. The way the players have their characters figure it out is through skill checks when the characters are feeling uncertain. Your job as DM is to dole out the information only when the characters perform a successful skill check or asks you the right question.
<snip>
Eventually the party will know what the DM knows. Be Patient.
This is a very prescriptive account of how to GM a RPG, even for D&D. It's not one that appeals to me either as GM or as player. There are many reasons why this is so, but one of them is the suggestion that all interesting information should be gated behind low-stakes skill checks.