Metabolism is a term used to describe chemical reactions in cells. In all chemical reactions there are the reactants (chemicals which react with each other), and the product (what you end up with at the end of the reaction).
There are two main sorts of chemical reactions: Anabolic and Catabolic. Anabolic means the reaction creates more complex molecules from much simpler molecules. Eg: using letters 'a' and 'b', A+B=AB. Catabolic means the opposite, using very complex molecules and breaking them down into simpler molecules. Eg: using letters 'a' and 'b', AB–>A + B.
Anabolic reactions usually need some energy to start reacting, whereas Catabolic reactions usually release energy. Metabolic activity doesn't happen spontaneously, it needs what is called an activation energy. However, sometimes the energy needed can too much for the cells to tolerate. Usually, the activation energy is heat energy, but reactions sometimes need a huge amount of heat to start, a heat so great that the cells would not survive it. To resolve this problem, cells have enzymes. Enzymes are long chains of Protein, but the long chains fold over one an other the create a very specific shape. Without the right shape, enzymes cannot work. Here is an analogy which should help you understand:
Imagine a hill with a little crater at the top. Within this crater is a boulder. You want to push the crater out so it can roll down (releasing energy) and shatter into smaller pieces (Catabolic Reaction). You have to push the boulder up over the crater's edge first however (activation energy), and this will take a lot of time and strength to do. So you are given a sturdy spade (enzyme). Using this spade, you can reduce the ridge, and thus reducing the distance you have to push (reducing activation energy needed).
There are also many factors which affect chemical reactions and enzymes. Two main factor would be temperate and pH. Back to our analogy, an affecting factor could be the rain. If it is raining, the spade (enzyme) will not work properly, with the water turning the dirt to mud. The mud would be very liquid and the spade will not successfully be able to clear away everything. Eventually, the little crater at the top would fill with mud, and then it will harden, making the pushing of the boulder over the edge (chemical reaction) impossible.
For the temperature factor:
If the temperature rises, then it is more heat as activation energy and more reactions take place. However, if the temperature continues to rise, the enzymes become denatured, i.e., they are malformed, and therefore cannot operate any longer. There is at first there is an increase in reactions, but then the number rapidly decreases as the enzymes are denatured.
The other factor is the pH. Every enzyme has a certain pH point at which they work the best. If the pH goes up or down, the enzyme will work less properly until it will stop working altogether. Imagine the spade we talked about earlier being bent back more and more until eventually it is bent all the way back to the handle, making it useless.
And that is all about enzymes and chemical reactions.