Hertz van Rental said:They are basically metal or plastic tubes that go through holes in instrument casings, walls, etc. You then feed a cable down the centre of the tube.
By various means the tube is then compressed to grip the cable and hold it in position in the hole.
They serve two functions:
To stop a cable moving or being pulled out.
To seal the hole that the cable passes through to stop water, vapour, etc. getting through.