Using more   labor while other inputs are fixed, the marginal product will eventually become less  and then this is known as diminishing marginal returns.

Increase in the marginal product of a variable input.

If a firm keeps increasing the amount of labor it uses, ceteris paribus, the total output will decrease.

The marginal product of variable input firstly decreases and then finally decreases again.

The cost advantage usually diminishes for each additional unit of output.