Simple Definition:

Telemetry is the process of collecting and transmitting data remotely [from a distance] using sensors and communication devices. It helps gather information about the performance, status, or condition of a system or object. For example, a fitness tracker collects telemetry data such as heart rate, steps taken, and sleep patterns, and transmits it to a smartphone app for analysis.

Very Simple Definition:

Telemetry is when information is collected and sent from far away using special devices. It helps us learn about how things are doing. For example, a watch that measures your heart rate and sends the information to your phone is using telemetry.