Divination is a way of looking at the likelihood of certain outcomes should you perform a certain action. Divination does not actually tell you the future as the future is constantly changing with every action you take. Think of it as standing at a fork in the road and trying to decide which path to take. You could do divination of some sort to figure out which path will take you where you want to go.
Many spellcasters do a divination before they cast a spell to see what the likely outcome of that spell will be. Will it help? Or will it do more harm than good?
There are many, many different forms of divination such as Tarot, runes, playing cards, coins, pendulum, crystal balls, scrying, etc. Perhaps the simplest form of divination is to take a single coin and flip it with heads or tails being a yes or no answer to your question.