OSM is OpenStreetMaps. This appears to be a server to locally mirror the ~90GB global tile set and serve them with your choice of style to OSM client applications.
I've always wondered why people keep adding the s. Is it marketing because Google Map sounds like a spelling mistake (missing 's after Google) and we're noticing the effect of that other brand, or is there something about OpenStreetMap that makes it seem like a plural project? People don't say Linuxes, Signals, or Androids
I think it's most likely that most of the other commonly used alternatives (most significantly Google Maps but also Apple Maps, Bing Maps, Maps.me) use the plural.