atime is short for access time or st_atime, from the stat structure.

In a unix filesystem, the atime represents the last time a file was read. The atime of a file is updated with the current time if any system calls are used on the file that create it or read it. The utime system call may set the atime.

The -u option of the ls command causes ls to use the atime (instead of the default of mtime) when sorting by (-t) or displaying (-l) the time.


extra info:
You can check the idle time of a user logged in by checking the access time of their tty device, as this will be the last time a read completed, and therefore, the last time they pressed return (or pressed a key if they are using a cooked mode shell.)