Much like the MODE option but this locks modes for the clients. This means that a user cannot remove the modes specified for that client class. For example, if you don't want users to be able to see wallops, you could modelock them -w.
/as client z-default set modelock +ix-ws
Modelocks are useful when you want to restrict some or all of your users. If you don't want your users able to turn off host mask munging (+x) then you can mlock it for them. Network operators can change their modes despite whatever modelock might apply to their client class. This means that mlocking something like -w, turns them into an oper-only mode.