Just when I think I'm getting a hold of the current system, you have got me with this one. What is IBLT?
Invertible Bloom Lookup Table.
Most of the block-generating peers have most of the transaction data that you have and will be using the similar rules to determine which transactions to include in a block. Given this, it seems wasteful to send the entire list of transactions of your block to each peer. Instead you send an IBLT which contains enough data for your peers to, with high probability, construct the precise list of transactions in your block.
At least, this is my understanding.