Look at Camera Expression
This page describes the Look at Camera expression tag in Cinema 4D. Tags like this are assigned to objects in the Object Manager to control their behavior automatically — in this case keeping an object permanently rotated towards the camera, the classic technique for camera-facing billboards such as flat trees or crowd cut-outs.
The Look at Camera Expression rotates the tag-carrying object with its Z-axis towards the camera so that you are always looking at the object head-on.
This is particularly practical if you want to place cut-out planes ("billboards") in the scene using the alpha channel in order to simulate "3D" objects that can be rendered quickly. This works very well for trees or people who are not in the focus of the scene, for example.
Here, a Plane object was assigned a tree texture and then cloned using a Cloner object. The planes are always aligned to the camera with the Look at Camera expression so that the texture can be seen frontally.