The securelevel is a security mechanism implemented in the kernel. Basically, when the securelevel is positive, the kernel restricts certain tasks; not even the superuser (i.e., root) is allowed to do them. At the time of this writing, the securelevel mechanism is capable of, among other things, limiting the ability to,
unset certain file flags, such as schg (the system immutable flag),
write to kernel memory via /dev/mem and /dev/kmem,
load kernel modules, and
alter ipfirewall(4) rules.
To check the status of the securelevel on a running system, simply execute the following command:
# sysctl kern.securelevel
The output will contain the name of the sysctl(8) variable (in this case, kern.securelevel) and a number. The latter is the current value of the securelevel. If it is positive (i.e., greater than 0), at least some of the securelevel's protections are enabled.
You cannot lower the securelevel of a running system; being able to do that would defeat its purpose. If you need to do a task that requires that the securelevel be non-positive (e.g., an installworld or changing the date), you will have to change the securelevel setting in /etc/rc.conf (you want to look for the kern_securelevel and kern_securelevel_enable variables) and reboot.
For more information on securelevel and the specific things all the levels do, please consult the init(8) manual page.
Warning: Securelevel is not a silver bullet; it has many known deficiencies. More often than not, it provides a false sense of security.
One of its biggest problems is that in order for it to be at all effective, all files used in the boot process up until the securelevel is set must be protected. If an attacker can get the system to execute their code prior to the securelevel being set (which happens quite late in the boot process since some things the system must do at start-up cannot be done at an elevated securelevel), its protections are invalidated. While this task of protecting all files used in the boot process is not technically impossible, if it is achieved, system maintenance will become a nightmare since one would have to take the system down, at least to single-user mode, to modify a configuration file.
This point and others are often discussed on the mailing lists, particularly freebsd-security. Please search the archives here for an extensive discussion. Some people are hopeful that securelevel will soon go away in favor of a more fine-grained mechanism, but things are still hazy in this respect.
Consider yourself warned.