The laser needs replacing, thats the problems with PS2's. I went through about 4 of them. They cost about �40ish o replace (i think cant really remember!) eBay is good place to find some (DONT GET USED ONES!)
My son had the same problem on his PS. The most likely cause is a dirty/dusty laser lens. If you are confident enough then unplug the PS and remove all casing screws to access the CD reader. Remember where the screws were taken from as some are longer than others. Removed the screws from the CD reader and you will then have access to the laser lens. Clean this with a lint free cloth and isopropyl alcohol. I got mine from http://www.shinyhardware.co.uk/products/showca t.asp/c=796 but you can get it in any pc shop. Replace the screws and it should work fine.
If your ps2 is still under warranty don`t open it as it will void it, and get it repaired by your dealer. If it`s older, open it up, but beware that you don`t damage the ribbon inside, and clean the little lens with a cotton bud and meths. Clean it gently in a circular motion, wipe off with a dry cotton bud and put back together again. It should then work fine.
what you have to do is buy a CD lens cleaner. it happened to my PS2 the lens is just dirty. You can buy a good CD lens cleaner at asda for about �5 or less maybe