Counters and timers are useful features available on some digital I/O. On T-series devices they are called digital I/O extended features (DIO-EF).
A counter increments an internal register each time the voltage on a digital input changes from low to high (or vice versa). This internal register can then be read at any time to get the current count. Some typical uses are measuring frequency (change in counts over some time period) or monitoring a tipping bucket precipitation (rain) gauge.