The Fjordur Portal Cave entrance is not too hard to find; it is in the center of the map at 40.3 - 57.5. Inside the Portal Cave, also known as the Place of Hidden Worlds, are three terminals that take you to other worlds. There is no danger in the Portal Cave other than what lies on the other side of the portal. There are three portals, one each for the Asgard, Vaneheim and Yodunheim realms. All but the Yodunheim realm are pretty normal as far difficulty is concerned in Ark Survival Evolved. Yodunheim is the most challenging because of the sever cold, so you will want to bring some high-end fur armor, primitive will leave you dead in a few minutes. On pc at least you don't have to use the Portal Cave, you can hold down the R key until a teleport options wheel appears. I would suggest using the cave at least once, since it is a pretty cool feature, and there is at least one rune in there.
Fjordur Portal Cave Coordinates
|Location of the Fjordur Portal Cave
If you are looking for resources, check Fjordur Resource Locations for more info.
If you are looking for dino and creature spawn locations check Fjordur Creature and Dino Spawn Locations for more info.