Sneak is a skill used in EQ to prevent many enemies from "hearing" you from behind. It will not work if you can be "seen" (front). When successful, Sneak will cause an enemy or NPC to appear "Indifferent" (/con) and not detect your presence. The Sneak skill is only available to certain classes (Bard, Beastlord, Monk, Ranger, Rogue) and races (Halfling, Wood Elf). Some classes or races have this skill innately at creation, while others will need to train it later.
When activated, Sneak reduces your movement speed. It is also important to know that Sneak can (and will) fail. You should always verify it is working at lower skill levels. You can do this by:
1. Checking movement speed.
2. Waiting for the Sneak button reuse timer to end (10 seconds) and see if it becomes available (pops up).
3. Conning (/con) an enemy from behind to see if they have become Indifferent to you.
You can find much more information about the Sneak skill by reading the discussion here at Allakhazam, or this thread at Monkly Business Forums. Be aware that some details may have changed since these posts were written.