There are a few ways plumbers clear blocked drains. The most common method is to use a plumbing snake to clear the drain. Plumbers can also use high-pressure water jets to clear the drain or use a chemical drain cleaner to dissolve the blockage.

What you're describing is a common plumbing problem called a "clog." A clog is usually caused by something like hair, grease, or food particles that have accumulated in the drain over time. Plumbers can clear blocked drains in a number of ways, but the most common method is to use a tool called a plunger. The plunger creates suction and force which can dislodge the clog and get the water flowing again. Other methods plumbers might use include snaking or hydro-jetting.