Well first off, the Infiltrator skill is only 2 points not 3 (it's 7.5/15 and 1/2 now). Your stealth level is basically how good you are at stealth. So increasing your stealth level means you could potentially get closer to enemies without being detected. This stealth level buff would stack with the skill Shadow Operative Elite which also increases your stealth level.
Stealth DETECTION level, like on the skill Scouting, is how good you are at seeing through an enemy's stealth. Scouting is basically a PVP skill.
Hope that clears it up for you.