Probiotics are good gut bacteria, and prebiotics are fibre that feed this good bacteria.
Each strain of probiotic (bacteria) has a different function, meaning one supplement on the shelf will have a different effect to the one next to it - which is why it's so important not to roll the dice and grab what's branded best.
Stick to natural probiotics like kimchi, sauerkraut, kombucha, kefir, miso, and tempeh. If you have a specific issue then research or consult a specialist/pharmacist on the correct probiotic strain to support the issue.
Examples of prebiotics are green(er) bananas, onions, leeks, garlic, and chicory root. Many prebiotics also cause the greatest digestive issues, so if you suffer from gut issues (garlic and onions for example) so it's important to consume as much as you comfortably can and increase this over time.
Couple these steps with a strategy to remove the "bad" bacteria in your gut and you're well on your way to finding a healthy balance.