Theres are good examples of ECL in the Forgotten Realms Campaign Setting if you have it. Anyway, ECL is effective character level that compensates for races that are powerful becasue of innate abilities. Such characters level slower than "normal" races.
Anyway the explanation. We are beginning play at level 4. Balin the dwarf has an ECL of 0 since dwarves are a normally powered race. At level 4 he has 6000 xp and needs 10, 000 to reach lvl 5.
A Sindar is ECL +1. At level 4, a Sindar PC would have 3 class levels in whatever class you choose plus his ECL modifier. A 4th level Sindar fighter for instance (lvl 3 fighter +1 ECL level) would have 10,000 xp and need 15, 000 to reach level 5.
Essentially PCs with ECL levles are "bumped" up the xp chart and progress slower. An ECL 0 PC could make 2 levels in the time an ECL 1 PC would make 1. Hopefully Im not being too convoluted in my explanation.