Turn the hubs to "lock" position to essentially lock the front axle together so it can recieve power from the transfer case when it is in 4wd.
some axles, like the Super Duty Dana 50, has a feature that u can still lock the hubs but still uses some sorta of activation when put into 4wd so u dont "need" to lock the hubs for mild driving.
there are a few styles of hubs...Auto hubs, manual hubs, drive flanges and...
here are some pics of when i rebuilt my front end:
the hub hat and hub
hub "hat" removed, exposing the internals:
hub "hat", cap screws and some of the internals:
more pics of the inside:
also note there is a large nut inside that needs to be removed that holds the wheel bearings together and on the truck. after this nut is removed with the special socket the whole rotor assembly can be removed reveling the spindle.
those pics are of a standard Dana 44 axle with factory warn/ford locking hubs.