What does the single line "_;" in OpenZeppelin's onlyMinter() function do?

Question

In onlyMinter(), there is a line with only _;. I'm having troubles adding a minter address, so I'm just checking the various contracts that go into ERC721. The function is

modifier onlyMinter() {
    require(isMinter(msg.sender));
    _;
}

What does _; do? Is it an error?

Answer 1

Modifier code is merged with the modified function. _ is the wildcard that tells the compiler where to put the function code relative to the modifier code.

Have a look over here for explanation, examples: Using a modifier as a function in Solidity

Hope it helps.