He may have been referring to freebsd's kern.securelevel variable, which seems to exist in osx 10.8, presumably having the same meaning. From fbsd man 7 security:
The security level can be set with a
sysctl(8) on the kern.securelevel variable. Once you have set the secu-
rity level to 1, write access to raw devices will be denied and special
chflags(1) flags, such as schg, will be enforced. You must also ensure
that the schg flag is set on critical startup binaries, directories, and
script files -- everything that gets run up to the point where the secu-
rity level is set. This might be overdoing it, and upgrading the system
is much more difficult when you operate at a higher security level. You
may compromise and run the system at a higher security level but not set
the schg flag for every system file and directory under the sun. Another
possibility is to simply mount / and /usr read-only. It should be noted
that being too draconian in what you attempt to protect may prevent the
all-important detection of an intrusion.
The kernel runs with five different security levels. Any super-user
process can raise the level, but no process can lower it. The security
-1 Permanently insecure mode - always run the system in insecure mode.
This is the default initial value.
0 Insecure mode - immutable and append-only flags may be turned off.
All devices may be read or written subject to their permissions.
1 Secure mode - the system immutable and system append-only flags may
not be turned off; disks for mounted file systems, /dev/mem and
/dev/kmem may not be opened for writing; /dev/io (if your platform
has it) may not be opened at all; kernel modules (see kld(4)) may
not be loaded or unloaded.
2 Highly secure mode - same as secure mode, plus disks may not be
opened for writing (except by mount(2)) whether mounted or not.
This level precludes tampering with file systems by unmounting
them, but also inhibits running newfs(8) while the system is multi-
In addition, kernel time changes are restricted to less than or
equal to one second. Attempts to change the time by more than this
will log the message ``Time adjustment clamped to +1 second''.
3 Network secure mode - same as highly secure mode, plus IP packet
filter rules (see ipfw(8), ipfirewall(4) and pfctl(8)) cannot be
changed and dummynet(4) or pf(4) configuration cannot be adjusted.
The security level can be configured with variables documented in
The immutable and append flags can be set on dirs or files using the chflags command. In order to lower or turn off the kernel securelevel you need to boot single user. Caveat: I don't know if osx makes full use of this feature, I only know that it's available.
Caveat 2: osx is actually based on the mach kernel. The userland stuff is from freebsd. Still, I see the kernel has this variable defined, so I'm thinking it might mean the same thing.