The clock represents the current time in hours and minutes in base 5. Each column consisting of 5 wooden spheres corresponds to a digit in the decimal representation of the hour and the minute. For example if the time is 17:42 the first column is 1, the second is 7, the third is 4 and the last is 2. In the lower part of the column count the number of spheres shifted up. If the sphere in the top part is shifted down add 5.
More fun facts about the clock can be found here abakus.in.tum.de