In the picture, there is a chemical reaction "taking place".
On the left, you have the reactants.
On the right, you have the products.
In the middle, there's a little curve that goes up higher than the products energy level.
Now there are quite a few things that drive a reaction, but right here we're focusing on energy levels.
Everything in the universe (that isn't living) tries to be as neutral as possible. Which means "having the least amount of energy as possible.
Now, what the heck is going on with the picture?
There is a little bar underneath the curve labeled "Activation energy".
That's showing us how much energy we need to make this reaction happen, and since it's going up (increasing) it means we have to ADD energy so this can happen.
Remember what drives a chemical reaction? Being as neutral as possible?
The reaction won't happen unless we add energy to it. And the easiest way to add energy, it to heat it up.
So since we need to add energy for this to happen... the universe would much rather just keep the Reactants just how they are. Thus, favoring the reactants (the reverse reaction).
I hope I helped :)
After I read this, I'll add a few extra things to it to make it less abstract and easier to understand if you're just totally lost.
