I cannot make sense of these lines and numbers, what do they represent?
These are positions of access points to underground water or gas pipes (for example a hydrant). The upper line denotes type and size. I don't know the Czech names, but in Germany the 'S' would be for a valve ("Schieber"). 20 is the diameter of the pipe. The lower two numbers show the position relative to the sign. The first one is 10.4m away from the wall and .3m to the left. I don't have a link to a description specifically about signs in the Czech Republik, but here is one in the German wikipedia: https://de.wikipedia.org/wiki/Hinweisschilder_zu_Stra%C3%9Feneinbauten