Meat shrinks after cooking.

"juices" usually mean liquefaction of fat/tendon/marrow, marinade (if any), and loss of water.

You "rest" the meat after cooking, because if you cut into it while it's still "hot", the liquid will run off the meat. You want it to cool down a bit so it'll stick to the meat, not on the plate/serving dish.

Brining (soaking the meat in salt water) is used to keep in more water in low-juice meats like chicken on a similar principal. Higher sodium retains water in the cells, so it'll be "juicier".

PS: Don't brine beef or fish. Too much liquids mean you'll be "steaming" the meat, instead of getting that nice char/crust. That's why cooking shows tell you to pat dry the meat and salt it.

never ever cut into the meat or poke a hole in it, test your meat by feeling it never pierce it

There's not a lot of blood in meat. It's mostly water, but with a meaty tinge. Probably some fat, too.

Could be both of those, and just water, too.

I'm a lifelong vegetarian and have never had meat so excuse the ignorance - I'm just curious. I love to watch anything on Food Network and such. Often I'll watch something and they will say oh you need to let the meat rest otherwise the juices will run out. What exactly are the "juices"? Is it blood from the animal? Marinade? Something else?