Unix time (also called epoch time or POSIX time) represents time as the number of seconds elapsed since January 1, 1970 00:00:00 UTC. That reference point is called the epoch.
Human dates and times tangle together timezones, daylight saving and calendar rules, making them awkward to compute. Unix time, by contrast, is a single integer independent of timezone, so it is simple to store, compare and do arithmetic on. That's why it appears almost everywhere computers deal with time: server logs, databases, API responses and file modification times.
For example, 0 means exactly the epoch (1970-01-01 00:00:00 UTC), and 1000000000 points to a moment in September 2001. Subtracting two timestamps immediately gives the gap between them in seconds.