A run level is a state of init and the whole system that defines what system services are operating. Run levels are identified by numbers.
Run level numbers
0 Halt the system.
1 Single-user mode (for special administration).
2 Local Multiuser with Networking but without network service (like NFS)
3 Full Multiuser with Networking
4 Not Used
5 Full Multiuser with Networking and X Windows(GUI)