What is a runlevel and how to get the current runlevel

A runlevel is a preset operating state on a Unix-like operating system.

To check the runlevel of you system you can use runlevel command with no arguments:

who -r

or

runlevel

Unix System V-style implement conventionally 7 runlevels. This runlevel implementation vary across many Linux distribution.

0 - System halt; no activity, the system can be safely powered down. 
1 - Single user; rarely used. 
2 - Multiple users, no NFS (network filesystem); also used rarely. 
3 - Multiple users, command line (i.e., all-text mode) interface; the standard runlevel for most Linux-based server hardware. 
4 - User-definable 
5 - Multiple users, GUI (graphical user interface); the standard runlevel for most Linux-based desktop systems. 
6 - Reboot; used when restarting the system.

By default Linux boots either to runlevel 3 or to runlevel 5.