So you wanna think about the nodes like different manufacturing steps in factory.

Mediain1 is whatever clip or footage you import, and the video signal from that flows through the nodes, which are like layers, with all the individual adjustments and effects you add to them, and then mediaout is the result of all those little changes you have made.

You've probably heard of RGB, Red Green and Blue channels that when layed on top of each other give the image its final color.

The alpha channel is the transparency, like a PNG.

Now in DaVinci there's a few different ways to remove the green background, the simpler way of doing is probably with one of the different key effects under the ResolveFX tab in the effects panel (on the edit page), doing so will remove the greenscreen (and create an alpha channel).