Multi-signature is a feature to protect against theft.
From securing your wallet
Bitcoin includes a multi-signature feature that allows a transaction
to require the signature of more than one private key to be spent. It
is currently only usable for technical users but a greater
availability for this feature can be expected in the future.
Multi-signature can, for example, allow an organization to give access
to its treasury to its members while only allowing a withdrawal if 3
of 5 members sign the transaction. It can also allow future online
wallets to share a multi-signature address with their users, so that a
thief would need to compromise both your computer and the online
wallet servers in order to steal your funds.
Traditional Bitcoin addresses are secured with a single private key. P2SH addresses can associated with three private keys. Depositing funds into is the same as depositing funds into a standard Bitcoin address. However, withdrawing funds from the address requires at least 2 of the 3 keys to sign.