Here's a ~90 second look at kerf to help you start getting your head around it. Once you do, you'll find it really easy! [VIDEO BELOW]
Intro to kerf (the short version):
When the laser cuts, it cuts down the center of a line. As it cuts, it removes around 0.005" of material. As it makes its way around a square, for example, it removes that amount on each side, ultimately making the square ~0.01" SMALLER than what you designed it to be.
An example of this would be a tab that fits into a slot.
if you're cutting a slot for something to fit into, it cuts that slot at ~0.01" LARGER than you intended it to be.
So if you're making a tab and a slot that are to fit together snugly, start by making the tab 0.01" larger -OR- the slot 0.01" thinner in height (or width, depending on which direction the material is oriented). ALWAYS make a test slot or two first because you'll often discover that 0.01" isn't exactly what you want, but it is a great place to start.