Naming the two sides
Chemistry has a name for each side of a reaction, and both are used on every page from here on.
Reactants
The substances that take part in a chemical change are called the reactants. These are what we start with.
Products
The new substances that arise from the chemical change are called the products. These are what we end up with.
The shape of every reaction
What takes place in a chemical reaction is reactants turning into products.
reactants → products
How to read the arrow
The arrow does not say "equals"; it says "becomes". It also shows the direction the reaction runs in — left to right.
An example
magnesium + oxygen → magnesium oxide
Here there are two reactants — magnesium and oxygen — and one product, magnesium oxide. A single reaction may have several reactants and several products.
A common mistake
Reactants are always written on the left and products always on the right. Swapping them states that the reaction runs the other way — which it usually does not.