An Operating System is the basic program that is executed by a computer or Video Game Console. (On consoles and mobile devices it is often called Firmware or ROM.) It provides for all user interaction and abstracts the underlying hardware in a way that programs (like the Tomb Raider Games) can access them in a standardised way.
Operating systems that support Tomb Raider games are for example Windows and MacOS X on computers. Almost all of the Tomb Raider Games and Lara Croft Games have been released for these two plateforms. Older Macs with PowerPC CPUs that were discontinued in the 2000s use the older MacOS.
Free operating systems like Linux need additional software (Wine) to support Windows programs, but there are compatibility issues. In the future we might see more gaming on Linux with Steam and their specially made Steam Box. Tomb Raider Legend, Anniversary, and Underworld are already playable as of 2014.