The term "Telemetry" comes from the greek roots 'tele', which means "remote" and 'metron' which means "measure".
Telemetry is the process of collecting data automatically from a remote location. Traditionally, this term has been used to refer to tracking via physical sensors built into a device or attached to the thing, place, animal, system or person that is being tracked. For example, animals are tracked to understand their migration patterns and natural phenomenon such as hurricanes are tracked to determine their strength and predict their probable paths.
However, recently another type of "telemetry" is being utilized en mass on users. This is when software on a computer (whether on a Restricted Platform or unrestricted platform) "phones home" to the developer or manufacturer with data about the users computer, configuration and/or usage. This is not necessarily an evil if it is done with the user's understanding and permission. This can benefit users by leading to better software thanks to the identification of bugs and software incompatibilities through crash reports.
But done without the user's permission or understanding, it is spyware. Unfortunately, this happens a lot. And rather than merely sending telemetry for debugging, it send information that identifies and tracks users, usually for the purposes of targeted advertisements. This data can be intercepted or sold to governments to track and identify citizens. This antifeature is rampant among apps on mobile platforms. Almost all commercial apps send usage data back to the developer without the users consent or knowledge.