SDL, or Simple DirectMedia Layer, is a set of C libraries that provides a standard programming interface accross platforms for accessing graphics, sound, and input hardware. Games written using SDL libraries can be easily ported to any platform on which the libraries are supported.
SDL has not yet been ported to any Atari platforms, however it is significant in the context of Atari emulation in that emulators written with SDL can be ported to nearly any major platform (such as Linux or XBox).
See Atari SDL emulators.
More information in Wikipedia