What do the brackets around processes mean?

Brackets appear around command names when the arguments to that command cannot be located.

The ps(1) man page on FreeBSD explains why this typically happens to system processes and kernel threads:

If the arguments cannot be located (usually because it has not been set, as is the case of system processes and/or kernel threads) the command name is printed within square brackets.

The ps(1) man page on Linux states similarly:

Sometimes the process args will be unavailable; when this happens, ps will instead print the executable name in brackets.

From the manual:

Sometimes the process args will be unavailable; when this happens, ps will instead print the executable name in brackets.