The run levels in Solaris are little different than what you know about run levels in Linux. So, if you come from Linux background this could be little confusing initially.
There are total 8 run levels in Solaris. Default run level is 3. At a given point of time, a system can be in only one run level.
0 Power Down State
To shut down the operating system so that it is safe to turn off power to the system. This will bring the machine to Open Boot Prompt (OK)
s or S Single User State
To run as a single user with some file systems mounted and accessible.
1 Administrative State
To access all available file systems. User logins are disabled.
2 Multiuser State
For normal operations. Multiple users can access the system and all file system. All daemons are running except for the NFS server daemons.
3 Multiuser level with NFS resources shared
For normal operations with NFS resources shared. This is the default run level for the Solaris environment.
4 Alternative multiuser state
5 Power-down state
To shut down the operating system so that it is safe to turn off power to the system. If possible, automatically turns off power on systems that support this feature.
6 Reboot state
To shut down the system to run level 0, and then reboot to multiuser level with NFS resources shared (or whatever level is the default in the inittab file).