The "suid" (Set User-ID) flag, is mostly meaningful when set for
a file containing an executable program.
When set, it means that when a User runs this file, the launched process
will have the permissions of the User-owner of the file, rather than
the permissions of the User running the program.
This flag is often used in order to temporarily give Users access to
files they cannot normally access.
For example, the X server is often installed with the "suid" flag set,
in order to allow the User to access the graphics devices (screen,
keyboard, mouse):