Passau is right on the Danube River so of course they have had to deal with flooding.
The lines on the wall are the dates of the floods.
From top to bottom they are:
1501
2013
1595
1954
1787
1862
1899
1895
2002
2025
1920
Not sure I believed the 2025 one as it looked like someone had just written it with a sharpie.