It means that you are advised to get involved in a situation because you definitely have a chance of success
I suppose a typical usage would be if a member of the opposite *** is being pointed out as someone who is interested in you. But it is used widely. For example, if you were thinking of app
The poms use it (especially in the east e.g. Essex) say something they like happens to them they will go 'get in!' which means yes or wicked or something. It originally came from football (when someone scored a goal they would yell 'get in' but now it also represents when something else great happens e.g. winning a raffle 'get in' or rocking up to a party and there being heaps of hot girls and boo
From what I have seen during my years in London, English people punch the air and shout 'come on' or 'get in there'. They do it when they are happy about something and their face turns pink/red like lobsters.
A guy in the bar won the pool and he keep on punching the air shouting 'COME ONNNNNN' while crying and spit coming out of his mouth. His girlfriend give him hug and he start bursting