Hi, below in the modifier onlyWhileVacant, what does _; in

Question

Hi, below in the modifier onlyWhileVacant, what does _; in

the 2nd line do?

contract HotelRoom {
enum Statuses { Vacant, Occupied };
Statuses currentStatus;

address payable public owner;

constructor() public {
owner = msg.sender;
currentStatus = Statuses.Vacant;
}

modifier onlyWhileVacant {
require(currentStatus == Statuses.Vacant, "Currently occupied."); //check room status
_;
}

modifier costs (uint _amount) {
require(msg.value >= _amount, "Not enough Ether proided."); //check payment amount
}

//updates currentStatus to Occupied. pays owner to book hotel room.
function book() payable onlyWhileVacant costs(2 ether) {
currentStatus = Statuses.Occupied;
owner.transfer(msg.value);
}
}