Input capture
A timer feature that freezes the counter value in the very clock cycle an edge arrives on a pin.
The naive way to measure a pulse is to read a timer inside an interrupt handler. The result is then only as precise as interrupt latency allows, and that latency depends on whatever happened to be running. Input capture moves the measurement into hardware: when the edge arrives, the timer copies its count into a capture register, and software collects it whenever it gets round to it. Precision becomes one timer tick regardless of load.
Two captured values give a period or a pulse width. That is how you decode an RC receiver, time the echo from an ultrasonic sensor or read an IR remote. The mirror-image feature is output compare, which underpins PWM. Hook it to DMA and whole pulse trains can be recorded without the core lifting a finger.