The legend colors don’t match to the background colors of the map. In the map there are areas witch have the following hex colors: #F2EFE9 and #EEF0D5. These colors do not exist in the legend.
And the query function do not response any information about the background type.
As well as the farmland and undefined areas that kartonage mentioned, looking along the whole length of the street there are also some residential areas, and an area of industrial landuse at the northern end.
One way: enable the data layer as PHerison described. This shows a blue outline around map features including the area you refer to. The outline is clickable. Clicking it shows the data (tags) for the feature on the left of the screen. This one is tagged as landuse = farmland.
Another way: use the query tool button as SomeonElse said. Move the pointer within the area close to the edge and click it. You will get a list of nearby features on the left of the screen, one of which is farmland. Move the mouse over the list and the relevant feature will be highlighted in red, confirming that farmland is the one we are looking for.
Another way (requires user account): hit the Edit button, select the feature of interest, and see what appears on the left.
I don’t know whether the German style also has one. Although it’s quite uncommon for online maps to have a key. For example Google Maps and Bing Maps don’t have one (or at least not one that’s easy to find).