A runlevel is a preset single digit integer that defines the operating state of a Linux and Unix-like operating system handled by init. Each runlevel allows for different combinations of running processes and vary depending on the operating system being used. The standard Linux kernel supports seven different runlevels, as shown below.
0 - System halt. 1 - Single user. 2 - Multiple users with no NFS. 3 - Multiple users under the command line. 4 - User-definable. 5 - Multiple users under a GUI. 6 - Reboot.
On a desktop computer using a GUI interface, your runlevel is a 5 and a server is likely a 3.