In this context it means what nanren says.
However it can mean other stuff in different contexts.
'Get down !' means take cover/crouch/hide behind something if someone is say, shooting at you. It can also be an order from a police offer for example which means 'get/lie on the floor'.
'Get down' can also mean a variety of other things.
Two blokes who want to fight. One can say to the other 'we gonna get down or what?' meaning 'are we going to fight ?'.
Then it can mean to have an intimite sexual encounter with someone.
'Me and Clarissa got down to business if you know what i mean.'
Then it can mean to 'start something'.
'Right, well that's lunch over and I have work to do so I'd better get down to it.'