That guy has paid £25 to keep the plate on 'retention'. This is much easier than him having it on his car and then swapping to you.
You'll then have to apply to the DVLA £80 to transfer it into your car (instructions on the back). They sort it all out, send you a new tax disk etc. I would get the plates made up for when it comes through and don't forget to change your insurance like I did!!!!