Let's try to make it easier:
functions followed by hashtag are functions of interpretation. For example: date # is a function that interprets a text as a date field.
But it's better than my explanation if you could read this:
A good way to learn about it is read and read Qlik Help.
Here it is a good documentation about Qlik's functions - https://help.qlik.com/en-US/sense/1.1/pdf/Script%20Syntax%20and%20Chart%20Functions%20Guide.pdf