Siduri is the tavern-keeper of the underworld. Where she comes from, and why she thinks running a tavern in the underworld is promising business opportunity, we have no idea.
We also don’t know what her regular patrons are like down there; we would have figured the underworld would attract a pretty unusual customer base, but apparently Gilgamesh’s haggard appearance when he shows up at siduri vahetus spells bad news, and she immediately bars the entrance to him. That said, given that Gilgamesh immediately starts threatening to smash her door down, she clearly is pretty good at sizing people up. He then tells her all about his sadness at Enkidu’s death and asks whether he will also die.
What happens next is interesting.
In the Standard Version (for more info on the different versions of the epic and how they matter to you as a reader, check out the «Writing Style» section of this module), Gilgamesh is really talking to himself—he doesn’t wait for an answer from Siduri. He does ask her where to find Utanapishtim, and she tells him how to find Urshanabi, who will take him to Utanapishtim.