What does this symbol mean in the context of an analog digital converter?
It represents a counter and the whole circuit is a dual-slope-ADC.

The line coming from the AND-gate is the counter's clock pulse input.
The line coming from the "Steuerung" block is the counter's reset input.
The "Steuerung" block should also control S1 and S2. 

For comparision see e.g. here (page 20):