LSASS.exe is the Local Security Authentication Server process. Basically it enforces Security Policy. If the process is taking up an inordinate amount of CPU cycles then I would first look at what security policies you have in place.
LSASS.exe has been hit by viruses in the past so you obviously want to make sure your Antivirus software is running and up-to-date.
Also, it is a core system file so don't try and kill it, because killing it will cause your computer to reboot.