suid (Set UserID) is a special permission set to executable files and directories on Linux and Unix-like systems. Items set as suid are ran under the context of the item’s owner. For example, if a program such as /bin/ping is suid root, when the user daniel runs ping, it is ran as the root user.
This is commonly used for programs such as ping that require raw sockets or passwd, chsh, and other programs that authenticate against or modify passwd/shadow files.