When you first enter a new fight you roll your ini check. This'll determine in which way the combatants react, so who'll go first.
So if the monter rolls higher he attacks first, if you don't have uncanny dodge you'll be denied dex bonus to ac, you're flatfooted. You'll go later.
If you go first you will have more chance to do damage.
In NWN it's really important to PCs who can sneak attack (lots of monsters are immune to sneak attacks
) or in huge fights as you either deal (if you're first) or receive (if you're later than monster) more damage, in huge fights being last means potentially receiving a lot of damage.