Probability is all about describing how likely something is to happen.
Some things are certain to happen. The sun is certain to rise tomorrow.
Some things are likely to happen. It is likely that you have a pet at home.
Some things have an equal chance of happening. There is an equal chance that a new-born baby will be a boy.
Some things are unlikely to happen. It is unlikely that you have four brothers.
Some things will certainly not happen. You will certainly not turn into a sheep overnight.
If something is certain to happen, then we know it will happen. If an event will certainly not happen, then we know it won't happen. But likely events, unlikely events, and events with an equal chance can either happen or not happen.