Because the kernel automatically loads needed modules, and unloads unused ones, these two commands are now obsolete. However, they can be used for debugging.
modload(1M) can be used to force a module into memory. The kernel might subsequently unload it, but modload(1M) may be used to ensure that the driver has no unresolved references when loaded.
modunload(1M) can be used to unload a module, given a module ID (which can be determined with modinfo(1M)). Unloading a module does not necessarily remove it from memory. To unload all unloadable modules and forcibly remove them from memory (so that they will be reloaded from the actual object file), use module ID zero:
# modunload -i 0
A future release might not include modload(1M) and modunload(1M).