A crate is one of the easiest things to render in 3D. It's a cube-shaped container, often made of wood.
Crates are often used in games to help define the boundaries of environments or to make them more interesting. This is traditionally because the geometric simplicity of a box makes it easy to render and simplifies calculations required for gameplay, such as collision detection.
Many games have often been ridiculed for the over-use of crates or boxes as they are implemented in many games and show lack of creativity in level design, they also tend to be considered an easy way to fill up a level or map of a game, prime examples of use of the said tactic are the Half-Life and Counter-Strike games.