In more detail
Because it counts from a fixed UTC moment, it is the same everywhere regardless of time zone; the zone only matters when it is turned into a local date. Unix time ignores leap seconds, treating every day as exactly 86,400 seconds, and moments before 1970 are negative. JavaScript and many APIs use milliseconds instead, so a 13-digit value is milliseconds and a 10-digit value is seconds.
Why it matters
Mixing seconds and milliseconds gives dates in 1970 or tens of thousands of years in the future. Systems storing it in a signed 32-bit number overflow after 2,147,483,647 seconds, at 03:14:07 UTC on 19 January 2038 (the "Year 2038 problem"), and wrap to a date in 1901. Timestamps are also central evidence in logs and file systems, where they can be altered (see Timestomping).
Example
1700000000 is 14 November 2023, 22:13:20 UTC; the same moment in milliseconds is 1700000000000. On Linux:
$ date -u -d @1700000000 '+%Y-%m-%dT%H:%M:%SZ'
2023-11-14T22:13:20Z
Convert in either direction with the Unix Timestamp Converter.