First, a quick primer: Probiotics, found in fermented foods and supplements, are "good" bacteria that are thought to help maintain digestive health and boost the immune system. Prebiotics are non-digestible fibres found in foods like onions, chicory, and garlic that feed the beneficial bugs in your GI tract, helping them function. The "waste" left behind from that process is what's known as postbiotics.
That sounds kind of gross. But actually postbiotics include substances such as organic acids, enzymes, and proteins called bacteriocins. All of these have advantages, and help do things like lower blood sugar or reduce inflammation. Some researchers even believe that at least some of the benefits we get from ingesting probiotics may be due to the postbiotics.
This is the early days of postbiotics. It's still seen as an evolving term, and experts aren't totally sure how they impact the body, although most scientists agree that they're good for the gut microbiome. Still, that means that as of now, there can be no firm advice given on how to tap into their benefits. There's no postbiotic supplement (yet).
Just... try not to think too hard about all those bacteria, eating their fibre and releasing their waste into your GI tract.