At its coarsest version, I would say "tuned to" is "within the realm of possibility for the players to defeat" or "not a cakewalk", and then we refine it from there. As in, if I have a party of level 8 PCs, I might as DM say...
- I will give the players whatever encounter is appropriate for the area/random table, including CR 1/8 creatures or a nest of CR 20 creatures. [This I would say is no tuning, and it is absolutely a valid style, I'm not discounting or discrediting it.]
- I will not throw CR 1/8 creatures at them nor am I going to throw CR 20 creatures at them, everything in between is on the table.
- I will give them a range of CR 6 to CR 12 creatures.
- I will follow the book's specific guidance for CR-to-party-level balancing on a per-encounter basis.
- I will follow the book's specific guidance for CR-to-party-level balancing on a per-encounter basis, and I will fill it with tactics that tickle the party's particular composition (minions for the fighter to mow through, artillery for the rogue to do clever movement tricks to get to, not include bags of HP if the party is all low DPS, etc.).
- I will [#5], and if a player drops or joins before the session starts, I will adjust the encounter(s) before we start.
- I will [#6], and if a player character dies or levels up before the combat starts, I will adjust the encounter before we start it.
- I will [#5, #6, and/or #7], and if the players look overwhelmed or bored, I will adjust the encounter mid-encounter.
I think that's a roughly ascending scale of "tuned to"-ness in my mind. I can't definitively tell you which if any of those challenges isn't "real", so I could see an argument that they're all "illusions", but I'll also readily and happily acknowledge that each DM probably has their own sweet spot, and that's fine.
For me, if they're wandering aimlessly in the wild it's #1 - they get what they get, no tuning involved.
If they're in an adventure or module etc. it'll be tuned to their vague level range - if the party is 7th-10th level then ideally I'll run them in a module designed for that level range. So I guess that's #3?
I haven't got the patience for #4, never mind the guidance always says to vary the encounter difficulties anyway.
I almost never look at or consider the specific makeup or lineup of a party, usually because when I write or select the module I have exactly zero idea what that will be. They each have several (or more!) characters and it's often difficult to predict who will get cycled in and out at any given time. That rules out #5 by default.
As for #6, if a player drops or joins permanently that doesn't change a thing. If a player misses a session the character carries on as normal, played by those who do show up.
#7 and #8 are right out.
One thing not on the list above that I do consider when writing/selecting modules is in-character motivations, and what they want to do both in and out of character.
For example, one player has a character who is very keen on finding a Dragon in hopes of taming it (good luck with that!) and-or looting its hoard; which means sooner or later that adventure will appear, and it'll naturally be level-appropriate as the characters are around the level at which Dragons are quite viable foes.
Another character in that party recently picked up a horrible curse which others in the party want to fix, which (if possible at all) isn't going to be easy and will no doubt involve some adventuring; the level-appropriateness of which may be somewhat more random.