(NOTE: Will be updating frequently)
A chemical reaction is a process where one set of substances is converted into another set of substances. So, reactants are converted into products. So, a chemical change occurs.
To represent a chemical reaction, there exists something called a chemical equation, which is a formula relating the reactants to the products, drawn with an arrow pointing to the products.
For example, nitrogen monoxide + oxygen -> nitrogen dioxide.
To make this into a chemical equation, we need to substitute the chemical formulas for names, so we get:
NO + O2 -> NO2
However, on the LHS (Left hand side) there are 3 oxygen atoms, and one nitrogen atom. On the RHS, there are TWO oxygen atoms, and one nitrogen atom. So, we have to balance the equation.
In a balanced equation, the total number of atoms of each element is the same on boths ides.
So, to balance that equation, you'd have to add a coefficient to NO and NO2, and it'd become:
2NO + O2 -> 2 NO2, there are 2 nitrogen atoms and 4 oxygen atoms on each side.