Skirret is a tool that builders and craftsmen use to make nice and straight lines when they need to cut or shape things like wood or metal. It looks like a ruler or a straight stick, but it has a little piece on one end that can move up and down. This piece is called a sliding adjustable vial or bubble level.
When a builder needs to make a straight line, they first put the skirret on the surface they want to cut or shape. Then they adjust the sliding vial to make sure the skirret is straight. Once it is straight, they use the skirret to draw a line, and that line will be nice and straight because the skirret helped them make it that way.
Using a skirret is like having a tool that helps you draw a perfect picture or write a perfect letter. It makes things look neat and organized and helps the builder make sure they are doing a good job.