It literally means that something does not have enough salt. If you say that someone is soso/sosa, it means that they are kinda unenthusiastic.

If it refers to a thing, it lacks salt. For a person, it means they're unenthused. (http://lema.rae.es/drae/?val=soso)

Definitely means not having any salt or only having a little salt (not enough). In English this is also sometimes translated as "bland" or "tasteless". In English, these words could infer that something doesn't have enough salt, but may also infer a lack of other seasonings or that it simply has no flavor. The soup was rather "bland". These steamed vegetables are "tasteless". Saludos!