Open the window, let fresh air in.

Meaning: This means to move the window so that new air can enter a room.

Choice of Words: Open means to move something so that there is a space or gap. Window is a part of a wall or roof that can be opened to let in light or air. Fresh means new and not used before. Air is the mixture of gases that we breathe. When you put them together, "Open the window, let fresh air in" means to move the window so that new air can come into the room.