Understanding transfer()

1function transfer(address to, uint256 amount) public returns (bool) {
2    address owner = msg.sender;
3    _transfer(owner, to, amount);
4    return true;
5}
6
7function _transfer(address from, address to, uint256 amount) internal {
8    require(from != address(0), "Transfer from zero address");
9    require(to != address(0), "Transfer to zero address");
10
11    uint256 fromBalance = _balances[from];
12    require(fromBalance >= amount, "Transfer amount exceeds balance");
13
14    _balances[from] = fromBalance - amount;
15    _balances[to] += amount;
16
17    emit Transfer(from, to, amount);
18}