P2SH stands for Pay to Script Hash. It is one method of performing multisig.

A partner's public key's hash value creates a P2PKH (Pay to Public Key Hash) which is used in combination with one's P2SH in order to send Bitcoin over a script hash.

When Bitcoin is sent to a P2SH address, the sender can use a script hash address that begins with a 3. This allows for multiple digital signatures to be required before the Bitcoin can be spent.