Sure!

The SUBSTRING part of the expression just returns the original string without the first and last 2 characters, as you noted.

Then the CONVERT turns the current datetime into a string using style 12, which is YYMMDD, as documented at https://docs.microsoft.com/en-us/sql/t-sql/functions/cast-and-convert-transact-sql#date-and-time-styles

Then the LEFT takes the leftmost 4 characters of that YYMMDD string, giving us a string that is today's date in  YYMM format.

That is the string passed as the first parameter STUFF, which starts at position 3 (the second parameter, which means the insertion is after YY), replaces 0 characters (the third STUFF parameter, and with no replacement means it's pure insertion), and then inserts the result of the SUBSTRING (that is the 4th parameter to the STUFF).

In summary, the expression strips off the first 2 and last 2 characters of the original string, and then inserts that between the YY and MM in a YYMM version of today's date.

Hopefully that helps!