Just had my car MOT'd and it passed but has got an advisory note which reads: -
Offside and nearside rear (floor) subframe mounting prescribed area is corroded but not considered excessive (2.4.A.2)
What does this mean and is it an easy problem to solve.

It means you'll probably have to get some welding done next year.

you can buy some stuff in a can that you can spray under your car to protect it. have a look in a motor shop and ask them about it. this might put it off for another year.

Rust needs cut out or you'll just keep chasing it.

get it fixed now and its one less worry for next year plus it could be far more expensive after another years corroding

im a welder and my advise is get it all cut out (remove all rust) like said above you be chasing it otherwise , get it sorted and at least you got the piece of mind , dont cover it with any treatments as it will cost you more in labour charges/time for a garage to remove it ,

I have been told by the garage that it has only just start to corrode, they have told me for 20 quid they can treat it and this will give me a couple more years. They will wire brush it, airline it down and then cover it in some treatment stuff.

yeah if its only just started, wire brush it off, and put some underseal sort of stuff over it.