Flossing your teeth everyday is very important to keep your teeth healthy and prevent gum disease, tooth decay and gingivitis as well as prevent tooth infections.
Even though brushing your teeth cleans your teeth on the outside and gets rid of bacteria and food on the outside of the teeth the toothbrush still leaves food particles behind.
Those food particles get stuck between your teeth which can then lead to bad breath, tooth decay, tooth infections, gum disease etc.
So it's important to brush your teeth and then afterwards use string dental floss and floss in between every teeth to make sure they are clean.
Flossing at least once per day is the recommended thing to do.