When I was in my 20's I went to a naturopathic dr and she introduced me to Acidophilus for the first time. You’ll usually see it as Lactobacillus acidophilus and it's basically one of the “good bacteria” your gut actually wants more of. It helps keep things balanced in your digestive system, which matters more than people think; because when your gut is off, everything feels off.
People usually take it to help with things like bloating, constipation, or just that general “something’s not right” feeling, especially after antibiotics since those wipe out the good bacteria too.
You can get it from supplements or foods like yogurt and other fermented stuff. It's one of those simple, foundational things that can help your body get back on track.