The main screen shows a list of counters and timers with their current values.
A light hint shows what will happen if you touch an item.
Touching a counter number will increment its value by the number shown in the hint (usually +1).
Touching on a timer will start counting time. Note that the main timer value doesn't change while it is running, but you can see the current time period growing in the hint below.
Touching on a running timer stops it, the running time period will be added to the main timer value.