The kit comprises the two straps, replacement pins, and a tool to help you carry out the job. It needs some patience, good eyesight, but ultimately it's not too difficult, and if I had to do this again it would take seconds.
I bought this because the buckle had come off one of the straps on my Garmin 405, rendering it almost useless (you really can't use it effectively carrying it in a pocket). With the replacement, the watch is as good as new again.
Before you try to replace the strap, search on You Tube for "replace Garmin 405 strap" and there's a short video showing you how to do it.