To unlock this achievement you must collect a total of 100 light seeds. Light seeds look like this and produce a mystical sound when they are near:
To find them I would highly suggest using the two following resources. A video like guide (it's not really a guide but gameplay footage of someone playing and getting all the light seeds) also a walkthrough when you need it. I used both of these and they both were very helpful.
Videos:
http://www.youtube.com/profile?user=escencede#p/c/F25B10961B...Walkthrough:
http://www.mahalo.com/Prince_of_Persia_Xbox_360_WalkthroughEnjoy!