ShadowChat is taken from ShadowCoin. Encrypted messaging from address to address using compressed public keys with xxhash and lz4 hashing and compression encryption.
http://www.shadow.cash/downloads/shadowcoin-p2p-em.pdf

Checkpoints are hashes of blocks in the blockchain for the wallet to check against as it downloads the blockchain to verify the chain is correct and the wallet is not on a fork.

(A fork is a deviation from the correct blockchain)