This is a traditional style of beer from Dsseldorf. Alt refers to the old style of brewing (i.e., using top-fermenting yeast) that was common before bottom-fermenting lager brewing became popular.
Many of the classic examples can be found in brewpubs in the Altstadt (old town) section of Dsseldorf.

A well-balanced, well-attenuated, bitter yet malty, clean, and smooth, amber- to copper-colored German beer. A malt character with notes of rich baked bread and nutty-toasty bread crust balanced with a hop aroma ofpeppery, spicy, floral, herbal character associated with Saazer-type hops.