What does the payable keyword do, and what are msg.sender and msg.value?

payable marks a function (or address) as able to receive the chain's native currency. msg.sender is the address that called the function; msg.value is how much currency (in wei) was sent with the call.

function fund() public payable {
    // msg.sender = caller's address
    // msg.value  = wei sent along with this call
}

Without payable, a function rejects any attached value — Remix shows payable functions as red buttons. Contracts, like wallets, have addresses and can therefore hold a balance. msg.sender and msg.value are globally available inside any function:

• msg.sender — the account that invoked this function (e.g. to record who funded).
• msg.value — the amount of native token sent in the same transaction (in wei).

To later send funds out to msg.sender, you must cast it to a payable address: payable(msg.sender).

Go deeper:

• Solidity docs: special variables and functions — msg.sender, msg.value, and the block/transaction globals.