Kerf is the amount of material that gets removed when cutting. With any sort of cutting whether by saw blade, water-jet, or laser cutting, some amount of material is removed to make the cut.
Laser cutting kerf is approximately .01cm wide - pretty small if you think about it!
This means if you draw a square 1cm wide, the square will end up 9.99cm wide and the hole will be 9.99cm wide The remaining .01cm was burned up by the laser.
This is not precise, the kerf can vary based on temperature, humidity, and a number of other factors. It's a pretty slim margin, so if you're working on something very particular, we recommend trying a sample first.