A mortgage is a loan the bank gives you, so you can buy a house, and you gradually pay back the loan. Over a period of like a million years, or at least it feels that way. I mean, I dunno how much of a thing it is with Outlanders because people's houses from home always pop up or they get their bank accounts full of gold so it's like super convenient.
And # is a number sign but it's also a hashtag. Used on social media sites like Twitter to identify a certain topic.