Effective character levels (ECLs) are a way to compensate for the traits of the more powerful races. The average drow, for example, has 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.