I do not understand what the $6$th line of code does here. I understand the output but not why it gives that output. What is the $"$f:$"$ for? Python is the language I am working in$.$

def alphabetical$\_$keys$($dictionary$)$:

keys$\_$as$\_$list $=$ list$($dictionary$.$keys$())$

keys$\_$as$\_$list.sort$()$

return$\_$str $=""$

for name in keys$\_$as$\_$list:

return$\_$str $+=$ f$"\{$name$\}$: $\{$dictionary$[$name$]\}"$

return return$\_$str