Headspace is the clearance in your chamber between your case head and the breech. It can be measured in several different ways, and there's a different story for each type
of cartridge. Some of the most popular cartridges are rimmed, non-rimmed and belted. Here's a good article (I wrote it) that explains headspace.
Headspace is different to someone that's installing a barrel, and special go/no-go gauges are needed. Chamber clearance (at the shoulder) is also important for shooters that want to make quality handloads. However, the handloader uses different tools to measure the chamber clearance that his
loads have in his