Lax = Salmon
Grava = preparing raw salmon (or other fish/meat) by keeping it in a mixture of salt, sugar and spices.
Salt and sugar keeps the fish from spoling, lactic acid bacteria preserves the fish. Back in the days this was done by digging a hole in the ground, cover the fish with spruce/fresh leaves and salt. Then lleaving it lying there for a couple of days. This also gives the origin of the word "grava" which in this context would bean "bury".
And is cognate to English "grave" as in a burial place! Other cognates are Middle English (and modern Dutch) "graven", and German "graben", both meaning "to dig". Ultimately deriving from Proto-Germanic *grabaną. "Groove" is also related. Interestingly the adjective sense of "grave" in English (as in "serious") is of unrelated Romance origin (cf. "gravid", "gravity", "gravitas").