Gonna radically oversimplify but everything in your gut is actually its own little ecosystem, influenced by your environment and genetics but mostly of course what you eat. And that microbiome actually has ways to talk to the brain and vice versa through the Gut-Brain axis, affecting your decisions and emotions. So if for example you eat only junk food, it will be bad for that biodiversity and your gut microbiome will be shit and it will make you feel like shit. But if you take probiotics, eat healthy and varied and whatnot, you'll have stronger more diverse microbiote, which in turns lead to healthier outcomes.

This is all still being studied of course and it's unclear if really either your brain or gut is in charge or both, and who affects whom and when, but they talk. But I feel like following the old adage of caring about what you put in your body is good enough. It just means the things we eat and ingest leave living traces in our body that can affect us after we're done with it. So yeah if you only eat shit you'll kinda feel more like shit.