Found this info for Champions Online, so maybe it is similar but I am not certain it is the same in STO. Anyway hope it might be helpful.
There are two types of Stealth in Champions Online - Perception Stealth and Aggression Stealth. In general, it is better to have Perception Stealth than Aggression stealth, though both are useful.
edit Perception Stealth
Hostile NPCs react to a player's presence at a certain distance (approx 50'). Increasing a player's Perception Stealth reduces this distance, allowing the player to approch closer before hostiles notice s/he is there. Note that NPCs do NOT attack at this distance - it is merely the distance at which they become aware of the player. If the player retreats, it is possible the NPCs will prefer to simply ignore the intrusion. The Statistic Intelligence counters Perception Stealth.
edit Aggression Stealth
Once a player enters a hostile NPC's Aggression radius, the NPC will begin to attack, and will call any linked NPCs to help. Aggression Stealth reduces this radius, allowing a player to approach closer without being attacked. The Statistic Dexterity increases Aggression Stealth.