What does the phrase locked and loaded mean?

This term originated in the military and meant that your M-16 had the magazine in place and was ready to fire. The order to "lock and load" means to get ready for action.

If you are "locked and loaded," it means you are ready for whatever comes next.

The term was in use before the M-16. Many western stories and movies have used the saying for rifles as well as for wagons. It also has been used as an old sailing term for the storage of goods being locked into place and loaded aboard.