If something screws in/together, it fits or fastens together by being turned.

This light bulb screws in. 这个灯泡是螺口的。

The steel rods screw together. 这些钢条是拧在一起的。

The two parts screw together, and then you can attach the umbrella.

Unfortunately, it leaks where the top and bottom parts screw together.

You'll need to screw the bolt in; that part is threaded.

That part screws in at the bottom.

See also: screw verb (METAL OBJECT)