Not all bacteria are bad bacteria. In fact, you're covered in bacteria now, all over your skin (sorry to any germaphobes reading this). But these bacteria stop bad bacteria from making a home on you and making you sick. Your gut does the same thing. POUNDS of bacteria exist in there, all of them helping you digest food, talking with your gut and immune system and making your body work.
Because of their importance, making sure they’re in perfect balance is essential, since not being in balance might be a path to a bunch of intestinal issues, all of which are NOT pleasant. So, make sure your guts will be healthy with the bacteria they need, which can be found in some probiotics.