Marklar is basically Mac OS X (pronounced 10) for the x86 architecture. With this Apple Computer, Inc. can live on if Motorola or IBM fail to deliver the PowerPC chips, that being the G4 or G5 chips.
Marklar has been steadily worked on by Apple by maintaining a PC compatible build of Mac OS X. This project has been going on since the late 1990's when Apple decided to move its OS from the Classic Mac OS, i.e. OS 8, 9 or earlier builds, to the UNIX-bases OS X.
Apple is currently not releasing these builds to developers or the public. There are reports of current Apple software, Final Cut Pro, iMovie, etc. able to run on the Intel and AMD based systems.
It's unsure if we will ever see Marklar in the near future but it is an emergency plan if Apple needs to fall back and maintain its presence in the computer world.
Sources:
eWeek
Macrumors.com