Interleaving is basically taking a block of samples from your signal and shuffling (interleaving) them around before sending them. This is so that if a short 'burst' interferer causes errors in a number of bits in the channel, the errors affect bits that were not actually next to one another when the signal was created. So when the data is 'unshuffled' (de-interleaved) then the error conrtol code can work on the errors more easily, becasue the errors are spread though the message, not obliterating any particular spot.