Need a little explanation on the mirror box

Hi guys, I have to admit I'm not fully versed with the technical ins and outs of my SLR's - so I was wondering if someone could help me with my question:-

I can see that behind the main mirror, is another smaller mirror, which as the main mirror raises, this smaller mirror folds up and goes flush underneath it. Does it have a particular name? What is going on there...

Also, if that mirror is dirty, can it affect exposure - I know anything on there wont show up in images, just in terms how the light and exposure is judged by the camera.

Sorry if this sounds confusing, or I have not explained it properly

The main mirror is partially silvered, meaning some of the light passes right through it to the second mirror.

The main mirror send the image up to the prism. This is the image you see for composition and is also the image used by the camera reflective meter for auto-exposure.

The secondary mirror sends the center of the image down to the floor of the camera where the autofocus sensor is located. This is the only purpose of the partial silver coating and the existance of the second mirror.

Brilliant, thank you for that JefferyG!