It's if used for defining the how the variable will be displayed or recorded in the String::format.
In this example %6.2f means 6 digits with 2 decimal places that is used for a floating value. So n= 91.2345 would be recorded as '91.23'.
If n=1234567.2345 then what is recorded would be '234567.23'.
Why I use 6 digits is beyond me because you only need 3 digits for the temp (for normal weatherstation temps you should never be above 999, god help us all).