Athley, when performers do a gig, they (or rather their management) produce a list of items that the venue must make available for the aritsts use in their dressing rooms. This is known as a rider.
It's usually a list of food and drink - and a few other basics like what joko has said above, but depending on the status and primadonnaishness of the artist the list can be long, and weird.
For example Mariah Carey's list might include such nonsense as...
16 red silk handerchiefs folded lengthwise,
A live Swan,
12 bottles of Cristal champagne 1975 vintage,
An albino dwarf called Eric wearing a blue hat,
13 jars of Marmite,
etc, etc
I think the tendency for everyone would be to start off being sensible but at some point you just think 'F*** it, I'll make whatever ludicrous demands I can get away with'.
That's the thing about being a big star, you lose a sense of normality.