What you are reading is the
Critical Damage Multiplier of a Greataxe. Most damage numbers don't show the x2 (or double damage) Critical Hits cause, because it's a waste of space. Due to the fact the Greataxe has a different Critical Damage Multiplier, it's noted in the text. Other weapons have this note as well, such as a Pick or or Scythe, which is x4. You may also notice something along the lines of "19-20/x2", which notes a larger Critical Threat (possible critical hit on rolling of 19 or 20).
Yeah, it's already been said, but whatever.

You can read more about it
at the SRD, but in summary the x3 damage for the
Frost Giant is only happening on a Nat 20 (which would be 9d6 + 39 if it confirmed) on one of his attacks (either a single from standard or two from a full attack...or more if you advance it). What you should be looking at is the CR 9 of the creature, noting that a party of 4 Lvl 9 PCs can feasibly take it (or something like that, though the system has its kinks). With 6 PCs it sound like they will have a good shot, but if all else, a numbers check at the
Encounter Calculator can help.
On if you should throw it at them depends on what they all have. For example you can't expect a party of adventurers to fight a
Bodak (or several) with no means to protect themselves or raise their saves against their gaze attack, right? Either a Cleric needs to be present or other forms of magic/abilities to raise Fort saves against such a thing (or at least helps in comparison to the obvious
don't look right at the creature technique). For the Frost Giant itself, obvious notes would be if the party has Fire spells, has means of buffing the Fighters to keep it away from the casters, and multiple weapons (make use of Giants strength and Improved Sunder for Great Justice). Of course, if you're using this Giant as a means to
teach the PCs to be prepared for such things, that's another matter entirely.
