I have seen elsewhere that it is a pun and can be loosely translated as either Frieren's at the funeral or Frieren caused the funeral. But I don't really know enough Japanese other than to say those are fairly liberal translations.

葬送 (sousou) literally just means funeral, フリーレン is Frieren and the の (no) in between usually indicates possession in some way.

I thought this is better suited for next week, but here you go.

葬送(Sousou) can be both a noun or a verb, as a noun meaning "Funeral Procession" (which is the wikipedia page that corresponds to the Japanese entry 葬送), or simply "Funeral" (but usually 葬儀(Sougi) or 葬式(Soushiki) is used to just refer to the ceremony).

As a verb 葬送（する）, usually it means "to attend a funeral", but it also means "to give somebody a funeral", and if you stretch that a bit, "to cause somebody to have their funeral".

So the translation "Frieren at the funeral" is using the "attend" sense of the verb, but there is another meaning to "Sousou no Frieren" that will probably be revealed in the next episode.