Effective character levels (ECLs) are a way to compensate for the traits of the more powerful races. The average drow, for example, has a several powerful abilities that make them more powerful than humans. To compensate for this, PnP use the concept of an ECL.
If drow have an ECL of +2, that would mean that theyâ€™re two levels lower than other races at the same amount of XP. For example, a human with 11000 XP would normally be level 5; a drow with 11000 XP is level 3. ECLs â€œlagâ€ your character behind by a certain number of levels, to compensate for your more powerful racial capabilities.