What do the letters K etc mean on the dollar bill?

They indicate the Federal Reserve district that distributed the bills.

All bills are printed by the Bureau of Engraving and Printing at facilities in Washington and Fort Worth. Each district only handles distribution.

Bills are printed for each Fed district as needed to meet the demand for currency within that district, then shipped there for eventual distribution to retail and commercial banks.

The districts are shown on a map at the Related Link, below. The numbers are the same as the letters in order; i.e. District 1 (Boston) is A on a bill, New York is B, etc.