What is the meaning of invoker level for a Warlock?

Hellfire Warlock prestige class (Fiendish Codex II: Tyrants of the Nine Hells variant, p. 89) mentions:

Invoking: At each level, you gain new invocations known, increased damage with eldritch blast, and an increase in invoker level as if you had also gained a level in the warlock class. You do not, however, gain any other benefit a character of that class would have gained.

What does the book mean by invoker level? The term invoker level is not mentioned in Complete Arcane under Warlock class.