(help!!) what does differentiation mean??

im doing igcse maths and im covering differentiation (in the context of y=yx^n) but i dont understand what it is, at all. can someone give me a really really simple definition as to what it is??

To put it in non-maths terms, think of this very odd question.
We are given a freeze frame of an F1 race video, and someone asks "how fast is the racecar travelling?" Well, as we learned before, velocity is distance over time. so we attempt to calculate it... except a freeze frame has time 0, so you're dividing by zero - no good.
But, we can approximate its speed by taking a very small time away from the freeze frame, say a second away, then we can do the whole distance over time thing. A second is too long - though, so maybe we could approximate this better with half a second, a quarter of a second, a single frame, etc..
And that's what differentiation (or really, the derivative) is intuitively - it's asking the question "how much does the output change when the input changes by a little bit." In fact, if you recall the definition of a derivative (the whole limit thingy), that's what it's saying - the ratio of a small change in the value of a function of x over a small change in x.
There is another one geometric interpretation, being the derivative is the gradient of the tangent line at point x. The intuition is similar to the silly racecar example, try if you could formulate it yourself!
Now in practice, calculating the derivative is applying a lot of rules and such, so the only way to get good is to do loads of them. Of course you could prove all of these funky rules by definition, but that's not important in GCSE, I think... (in fact, good luck proving the chain rule).