This is somewhat of an informal way of saying, "You understand, don't you?" or "You do understand, right?" When you say this expression, the assumption is that what you're saying is not too difficult and that the person you're talking to should not have a hard time understanding. You usually say this expression after you explain a procedure. You tell someone what needs to be done, then you say, "You get the idea." Sometimes, there's still more you can say, but you sense that the person you are talking to already understands, so you stop and you say, "You get the idea."
If you've been following this blog for a while, you've probably seen this expression used quite a few times, usually after I explain something. When I do that, what I mean is "here's the explanation; it's not too difficult; now, you should understand it."
A: When do you have time to study English? You are so busy.
B: I find time.
A: Yes, but how? You work; you take care of your family; you do everything at home; ...
B: I find time. I think it's very important to improve my English.
A: But how do you do it?
B: Well, I turn on the TV when I'm cleaning the living room or I listen to the radio; It's important to listen to English even when it's still difficult to understand it. I try to read one short newspaper article on the train everyday, and I underline the words I don't know. I try to sing English music. I try to speak English everyday. ... You get the idea.
A: I admire you. You're very dedicated.
B: Thanks. Like I said, I think it's very important.
A: I should do the same thing you're doing.
somewhat - kind of; sort of
assumption - what you believe or accept as true
Alright folks, enjoy the weekend.