It's an age related thing. You can have expensive tyres and they'll go like that after 4 or 5 years. It'll tell you when the tyre was made on the wall. Four numbers together in a little box. E.g. 3706 etc. That'd be made in the 37th week of 2006. I've heard you're not really supposed to keep tyres over 5 years anyway.
Buy cheap, buy twice TBH.
My rears say 1001.