In a nutshell, the pistons in your car are the little things that go up and down, up and down, up and down real fast to make your car go...oil shoots thru these to keep them lubed. If they are not lubed, they eventually get hot enough to bend, and then they don't fit in their little hole, and then really bad things happen to you car.
It is recommended to change the oil in your car every 3000 miles or 3 months because oil loses viscosity over time. Viscosity is its ability to lubricate. So old oil will allow more heat from friction to build up in the engine and the engine will run hotter. Insanely hotter? Probably not. But heat is the enemy. If things get hot enough, they expand. And if they expand enough, the gaskets that hold things together (you know, like, things that seal oil in one place and coolant in another) break their seals. And then oil leaks out, or, worse, water leaks in...and if water leaks into your oil, that is supposed to be keeping the friction down in your happy little pistons, guess what happens? Not cooling and lubing. Water that boiled off doesn't lube anything...
And IIIIIIII didn't have 2 years of mechanic school, and IIIIIIIII am a dumb blond girl, and IIIIIIIII STIIILLLLLLLL GET THIIISSSSSSSSS!
ARGH!