Before filming starts everyone will run through the scene whilst the camera, lighting and sound set up. Once the director is happy with your actions and performance the process of filming begins.
A Scene will be broken down in to a series of camera angles and each one of them is filmed in turn until there are at least two good attempts (takes).
Every shot has a number shown before the action starts. This is so that it can be identified and a record kept of whether there were any problems with it. Other people use clapper boards. The person who does this job is known as the Clapper Loader.
Before every shot a countdown from 5 is used. This is so that everyone has time to concentrate before the action actually starts. 2 and 1 are silent so that it is not picked up on the sound recording.
There are many reasons why filming may have to be stopped. The camera could be out of focus or miss action, the microphone could dip into shot, there could be an unwanted noise, the lighting could be wrong (too much, too little). You might not be standing in exactly the right place, you could deliver the wrong lines, miss words out or just get it wrong. There are all sorts of reasons why a shot could go wrong.