It's an approach to growing single cell protein (e.g. products like Quorn) that uses hydrogen oxidizing bacteria with hydrogen from renewable-powered water electrolysis.

https://en.wikipedia.org/wiki/Single-cell_protein

https://en.wikipedia.org/wiki/Hydrogen_oxidizing_bacteria

I have more detailed links in this old comment of mine from when Solar Foods appeared on HN: https://news.ycombinator.com/item?id=18355483