Ugh, what do you mean? Where do you see that? Why are you asking in Speech Codecs forum?
The only tip that one can give you for such a vague question is that "lookahead" = "look ahead" (looking at something that is incoming). Perhaps Wikipedia definition has a good answer for your badly stated question.

I don't know of a specific meaning in the context of Speech Codecs.
AFAIK in general audio it means "looking or listening to a signal before it's actually needed". This will give you some extra time to decide what to do with it.
Example: in the old days of LP vinyl cutting, the analog mastertape was played back on a special recorder with 2 playback heads. The first head was used by the mastering engineer to listen to the tape (and see the peak level on a meter) and make small corrections if needed. The second head was used to feed the cutter. There was a delay of (IIRC) a few seconds between them (depending on the tape speed) so the engineer had some time to react (usually to reduce level peaks to prevent distortion).

Hope that answers your question.