This monument stands in tribute to Kaiser Wilhelm I of the German Empire and the first Emperor.
This monument is sculptural ensemble featuring an equestrian statue of the kaiser, with four allegorical statues spread over the park and it's not obvious that the five sculptures form a group. The equestrian statue of the kaiser is fairly typical and features the kaiser dressed in military uniform wearing the Prussian Pickelhaube Helmet.
The four allegorical sculptures symbolise the major achievements of his reign, "International Transport and Communications", "Pension Law", "Measures and Coinage" and "Constitution".
It's far off the main tourist circuit in Hamburg, but worth a look if you have the time.