UNIX timestamps are often based on the count of seconds since the beginning of the day Jan 1st, 1970 (UTC).
Timestamps are used to distinguish otherwise identically named items, as well as a tag events (e.g. in log files) so they are not just sequentially ordered but can be related to real-time events.
As part of file names, timestamps are often used as the system provided timestamps (available via
stat) can change if files are later updated.