I am looking at the residue sequence for a protein that I'm studying for a class and I have a few questions.

I understand the regular amino acid codes (CYS, ARG, LEU, etc...).
What is NAG? I see one sequence "NAG NAG NAG NAG" (resisting off-topic joke)
What is HOH? I see about 107 "HOH" elements in a row. They are all colored red. What is that?

On the sequence number bar, I see one break represented as "//", and one gap in the numbering. 130-133 is "NAG NAG NAG NAG", and 140-246 is all HOH, but there is seemingly nothing at positions 134-139. Why is that? What is the difference between a "//" break and a numbering sequence gap?