These are also known as God Rays and Sun Shafts.
"Shafts is a graphics effect commonly used in realistic looking games. It is similar to how, in real life, when you can see sun shafts shining through your window, for example, in the early morning. This is used in Experimental Rust on trees, buildings, and other physical objects to make the game look more realistic. If you want to improve frame rate, this should probably be turned off. This can be toggled on or off via the Settings panel (F2 key by default.) You can only access the settings panel while in a world, not on the main menu.
Note: Shafts are ON by default.