It’s known as surplus value. The amount of value created by a worker beyond what they’ve been paid to create it.

If a worker is paid $10 an hour and produces $100 of value in that hour then they have created surplus value of $90. Surplus value is kept by the company as profit. Socialists argue the amount of surplus value taken by companies has grown exponentially since the industrial revolution where a single worker could produce massive amounts of value in a small amount of time. Wages have not increased to compensate for the massive increase in surplus value.