I feel really stupid asking this, because there's probably a simple answer but what are concessions? My guide book mentions them in relation to a bunch of the sites for London. Thanks for your help!

Here in the States concessions are usually stands or booths where food & drinks are sold. I am not sure if the reference is the same in England.

if i remember correctly it is special discounts ( students, seniors members etc)
on tickets.

Yep - concessions are special prices for senior citizens, disabled, whatever.

Definition of which categories of people qualify for 'concessions' [short for concessionary price(s)] differs among establishments, so you need to check out. Senior citizens (either 60+ or 65+), students (sometimes with age limit, say under 25 plus ID), disabled (with or without official ID), children (ages), unemployed and others on benefit, and so on.