The ECL is a number added on some subraces (Usually seen as +1, +2, etc) and stands for the following: Effective Character Level. It is a number relative to the starting strength of certain special subraces. Alternatively, it is called "Level Adjustment". It affects gameplay as follows:
ECL and Racial Power
Some subraces have added abilities surpassing the basic races in Neverwinter Nights. This racial power would ordinarily make a level 1 creature of the race in question far superior to a level 1 character of a race without such bonuses. To counter this, special subraces are given an effective character level, or ECL. This number is in effect, the number of levels needed for a level 1 human to be on par with a level 1 character of the given subrace. This number does not always balance perfectly with the relative power of the subrace; but it also can denote that the race is less effective as a Player Character due to its rarity, or other factors.
ECL and Leveling
The effective character level factors into how a character with a subrace can be leveled up. Whenever experience would be gained, the ECL causes the character to receive the same amount of experience points that a character of the same level plus the ECL number would receive. They are already as powerful as a character of higher experience, and wouldn't benefit from gaining it as much. Ultimately, the character will gain less experience for each monster slain and level more slowly as a result. A simillar experience penalty for multiclassing in several base classes can apply and stacks with the ECL penalty, and can hinder a multiclassed subrace character severely.
The ECL does not cause the character to start at a lower level than they would ordinarily, but does prevent the character from advancing to the maximum of level 30. A character with an ECL of +1 would only be able to reach level 29, for example, as at that level they are estimated to have the power of a level 30 character.
Subraces with ECLs
A list of Subraces with Effective Character Levels and their level adjustments: