I saw the following code in one example:

quotient[values_, times_] := 
 First[Differences[values]/Differences[times]]

mm = MovingMap[quotient[#BoundaryValues, #BoundaryTimes] &, 
  media, {60, Right}]

I understand it's defining a Pure function quotient which take 2 lists and returns the difference of each element with the next. I can't get why I also need First but the hardest part is in the next line.

I found that #name is called Slot and represents the value associated with key "name" in an association in the first argument.

body& is used to define a pure function... but quotient is already a function...

I can't get the syntax.