To "head on" needs 2 physical locations and movement between them. "The man was standing on my verandah (location 1), after he finished his drink I told him to head on (movement) into the house (location 2)." "I told my wife I was going to head on out (movement) to the yard (location 2)" location 1 in this example it not actually stated, but because you are heading out to the yard it is implied you are currently in the house, you could not head on out to the yard if you were already in the yard (only 1 location), also in this example "out" means you are inside and going outside. You cannot say "I will head on out to the living room", though you may be outside the living room you are still inside the house, so you would use IN or INTO. "My mum is telling me to head on up (movement) to my room (location 2)" again in this example location 1 is not stated, when you move up in a house it is short for "upstairs", so location 1 is implied as being on a lower floor than the bedroom. You can also have a head on collision. This where either 2 vehicles (or people/animals etc.) will crash into each other where both vehicles are travelling towards each other and facing each other or where 1 vehicle crashes in to another (or a person, or a signpost etc.) from its front.

Hi Shirley "Head on" simply means to get going, in other words, the user wants you to start moving in a certain direction or towards a certain location. It is a very informal term, but still a command, in other words, the person has to do it now.