This is a interesting position because it can mean different things depending on what type of set you are on.
Originally, a loader¬† on a film set would load the film into the magazine for the camera.¬† They would also be responsible for the clapboard (slate or sticks) that describe the shot and helps the editor sync sound.
Today not all films are shot on 35mm, they use P2 cards or SD cards instead.¬† Because of this there isn’t really a need for anyone to be specifically in charge of loading the camera.¬† To keep up with the times the loader position has merged with other camera department positions.
On film sets in order to allow multiple people to watch what is being recorded on the cameras monitors will be set up for the director, producers , DP and script supervisor.
The person that operates, controls and manages this feed and makes sure it is set up and working is called a video assist.
To get to this position, you need to start out as a production assistant.¬† On set make friends with the current video assist and find out how they got there.
You should learn all you can about technical side of filmmaking, for example; learn about all of the cameras, what monitors are used, how to set them up, and most importantly how to troubleshoot when a feed is lost.