A sterile seed starting medium, for certain values of sterile, won't have any stray seeds that will pop up and compete with your seedlings, and it won't have any plant pathogens like the fungus that causes damping off. These mixes are sometimes called "soiless" too, because they're mostly made of peat or other non-soil substrates like coir.
I really liked the Happy Frog potting soil that I used last year, and it was anything but sterile. It was jam packed with beneficial organisms and my seedlings thrived. Everybody has their own favorite, and some mix their own. My advice is to stay away from teh Miracle Grow type potting soils that have high nitrogen, time released chemical fertilizers in them, but there are those here who would argue that with me.
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