To let in means to allow someone or something to enter a space. When you let in a friend who's knocking at your front door, you open it and welcome them inside. A window screen lets in fresh air while keeping bugs out. A movie theater usher lets in ticket holders when the doors open.

The phrase can also mean accepting or admitting something more abstract. When you finally let in the truth about a disappointing situation, you stop denying it and face reality. A teacher might let in late students to class, showing mercy despite the rule being broken.

Sometimes we talk about letting in light, sound, or other things that pass through barriers. Heavy curtains block sunlight, but sheer curtains let it in. A crack in a window frame might let in cold air during winter.

The opposite is to keep out or shut out. When you close and lock a door, you're refusing to let someone in. The phrase suggests control over a boundary: you decide what crosses the threshold and what stays outside.