Tier lists originated in video game culture pretty much. It's a way to try to subjectively rank things within a game by its viability. The s tier thing comes from Japan I believe, where in some academic cases they give an S grade for excellent performance. In the context of tier lists the reason there might be a S+ tier is because some characters might be really really good and are in S tier but some characters are just way better than everyone else and are in a league of their own.