A drain lets water out. Sinks, bathtubs and some other locations in homes have drains. Most drains rely on gravity to get the water moving, but some will have an associated water pump. Drains may have a grid to keep items that shouldn't wash away from going through. Some drains also have stoppers so you can fill a sink basin or bathtub.
When the pipe leading from the drain gets clogged with stuff that washed down there are several ways to clear the clog. One is a commercial drain cleaner that you pour down the drain. Use these with caution as they tend to be caustic. Use a physical device to push or pull the clog out. Then there are homemade ways to clean a drain. One is to pour baking soda (probably about 1/2 cup) down the drain and follow it with about a cup of vinegar and then hot water. (Boiling hot water is good.)