The definition
A Unix timestamp counts the whole seconds that have passed since 1 January 1970, 00:00:00 Coordinated Universal Time (UTC) — this fixed starting point is called "the Unix epoch." Timestamp 0 represents that exact starting moment; every timestamp after it counts upward, one per elapsed second, with no ambiguity about timezone since the count is always relative to UTC.
A worked example
Timestamp 0 converts to Thursday, 1 January 1970, 00:00:00 UTC — the epoch itself, by definition. Timestamp 1,700,000,000 converts to Tuesday, 14 November 2023, 22:13:20 UTC — a specific, unambiguous moment reached by counting 1.7 billion seconds forward from the epoch.
Why computers use this instead of a calendar date directly
A single integer is trivial to store, compare, and do arithmetic on — checking whether one moment is before another, or how much time elapsed between two moments, is just integer subtraction. A calendar date-and-time string (with its month names, variable month lengths, leap years, and timezone offsets) is far more awkward to do the same arithmetic on directly, which is why most systems store time internally as a timestamp and only convert to a human-readable calendar format at the point of display.
Why it's tied to UTC, not a local timezone
Because the timestamp counts from a fixed UTC moment, the same timestamp always represents the identical instant everywhere in the world — there's no "which timezone is this timestamp in" ambiguity, unlike a bare date-time string. Converting a timestamp to a human-readable local time is a separate, additional step (applying the viewer's timezone offset) that happens after the universal timestamp value is already fixed.
Common places you'll encounter Unix timestamps
API responses and database records (many systems store event times as timestamps internally), file metadata (creation/modification times), URL query parameters for date-based filtering, log files, and JWT (JSON Web Token) expiration fields. Recognizing a 10-digit number in one of these contexts as a likely Unix timestamp (in seconds) — and knowing to convert it before trying to read it as a date — is a genuinely useful skill for anyone working with raw data, logs, or APIs.