It can also mean to continue doing something, without stopping or being distracted.
Here are some examples:
* "Stop complaining and get on with it!" (meaning: Start doing whatever task you're supposed to be doing)
* "I'm tired of waiting, let's just get on with it." (meaning: Let's start the activity without further delay)
* "I'm not going to let this small problem stop me, I'll get on with it." (meaning: I will continue working despite the obstacle)
The idiom is often used in a slightly impatient or forceful tone, but it can also be used in a more encouraging way. It basically means to take action and move forward.