Ok, let me take a stab at this.
A car’s engine works by exploding gasoline. The exploding gasoline forces a piston down which turns the crankshaft, just like your legs pushing down on the pedals of a bike. Unlike a bike, the engine doesn’t have anything (like your legs) to pull the pedals back up, so the way that the pistons get back up to the top is from the momentum of the engine parts moving. It’s actually a bit more complicated than this because the piston has to push out the exhaust gasses too, but the point is that the engine can keep itself going as long as it is spinning at a certain minimum speed. If it spins too slow then you won’t have enough momentum to push the pistons back up and around and the engine dies. This is how people stall out with a clutch, they let the engine speed get to low.
You need something in the car that can disconnect the wheels from the engine when you stop so that you don’t stall out the engine. In a manual transmission, this is the clutch. There are two plates, and when you push on the pedal one plate moves away from the other and the wheels are no longer attached to each other. Push your hands together really tight. Now move one hand. Your other hand will move with it. Same idea.
When they created automatic transmissions, they needed something that would do the same thing as the clutch but wouldn’t require the driver to operate it. There have been a few “automatic clutch” cars over the years, but the main thing they use is called a torque converter. A torque converter is like two fan blades with a bunch of fluid in between them. If you spin one, the other one is going to want to spin. Because there is fluid in there, you can stop one of the fan blades (the one attached to the wheels) and the other one (attached to the engine) can still spin. It’s just going to slosh around the fluid. If you let go of the brake, the fluid pusing on the fan blades attached to the wheels will make them want to turn, so you have to keep your foot on the brake at a stop light even if you are on level ground, unlike a stick shift where you don’t have to.
The good thing about a torque converter is that it allows the wheels to stop without stopping the engine because it has kind of a sloppy connection between the engine and the wheels. The bad thing is that this kind of sloppy connection is always sloppy, even when you are driving down the highway at 55 mph. A stick shift (manual) transmission, on the other hand, has a very tight connection between the engine and the wheels as long as you don’t have the clutch pedal pushed in, so while you are moving you get a much more efficient transfer of energy between the engine and the wheels. Newer cars will often have what is called a “lockup torque converter.” What this does is when the car is moving, it locks the two fan blades together in the same way that a clutch locks the engine and wheels together (although the mechanics of what does this are different). This way you don’t lose power to just sloshing fluid around in the torque converter when you are driving down the highway. It’s kind of the best of both worlds. You get the stopping advantage of a torque converter with the rolling advantage of a clutch type system, all inside a transmission that the driver doesn’t have to operate.
