picture it like this:
you're riding a ten speed bicycle (yea yea, I know, that was a ****ING long time ago)
you're starting at a stop, which gear is it on?? the biggest gear, right?? Now step on the pedals with all your might, and what happens?? it goes forward, FAST!!!!!!
Now, picture this...you're cruising at a comfortable pace in the smallest gear, your speed gear, and you've JUST finished cycling up the biggest hill of your life. Suddenly, out of ****ing no where comes the two biggest dogs in your life, and they're about to do some serious damage.
What do you do??? You slam on the pedals HARD to go, but you're in the smallest gear, so what happens?? It's like hitting a brick wall, and is HARD to pedal......
same thing with a car,....... so when you're in the higher gears, and you step on the gas, and it just revs, that's the clutch saying, "I'm too pooped to do this anymore, it's too hard to turn the motor"
sorry for the stupid description, that's just the best way I could describe it after drinking a few.