Probiotics are good bacteria that live in your digestive tract. You are born with a balance of good vs bad bacteria. Over time with Antibiotics , stress, etc your good bacteria is depleted. The good bacteria when in balance is 75% of your immune system So getting it back in balance is important.
You can get good bacteria from eating REAL Yogurt but you don't get enough and most yogurt is mostly Sugar anyway. So it is good to get those good bacteria from a supplement.