German license plates are fun to read:
the first letters are an abbreviation of the city or area, where the car comes from, the other letters and numbers are just to identify it.
HH is short for Hansestadt Hamburg, but you will also find some other rather common license plates in Hamburg:
HH = Hamburg
PI = Pinneberg
SE = Bad Segeberg
ROW = Rotenburg/Wümme
well, I could go on for hours and hours, but you might like to find out yourself!