man chmod

STICKY FILES
On older Unix systems, the sticky bit caused executable files to be hoarded in swap space. This feature is not useful on modern VM systems, and the Linux kernel ignores the sticky bit on files. Other kernels may use the sticky bit on files for system-defined purposes. On some systems, only the superuser can set the sticky bit on files.

STICKY DIRECTORIES
When the sticky bit is set on a directory, files in that directory may be unlinked or renamed only by the directory owner as well as by root or the file owner. Without the sticky bit, anyone able to write to the directory can delete or rename files. The sticky bit is commonly found on directories, such as /tmp, that are world-writable.