I copied this extract from man chmod
On older Unix systems, the sticky bit caused executable files to be
hoarded in swap space. This feature is not useful on modern VM sys-
tems, 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.
When the sticky bit is set on a directory, files in that directory may
be unlinked or renamed only by root or their 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.