When you send bitcoin, you aren’t really moving it between addresses. The Bitcoin is essentially stored in a digital vault where it stays regardless of the owner. What you’re actually doing is changing the locks to fit the recipient(s)’s key rather than your own.
Cryptocurrency wallets will either require one or several signatures (or ‘keys’, as in the example above) to gain access and sign and send a cryptocurrency transaction. If several signatures are required, it’s known as a multi-signature (multisig) wallet. If it’s one signature, it’s known as a mono-signature (monosig) wallet. The former requires only one signature, or key, to unlock a wallet, hence mono, meaning single, and signature (key). Monosig wallets are by far the most common on the Bitcoin blockchain. Wallets that require several signatures are called multisig wallets.