When a new block is found, most of its transactions are already in your mempool. Sending them all again would waste bandwidth. A compact block (BIP152) sends only what is new.
What it contains
- The 80-byte block header.
- A 6-byte short ID per transaction, derived from SipHash keyed with the block header and a random nonce.
- A few "prefilled" full transactions, at least the coinbase transaction, which the receiver cannot already have.
The receiver matches short IDs against its mempool, asks for any missing transactions, and rebuilds the full block.