As computers became useful utilities, a need to track time was necessary. Computers interacting with one another needed a way to share a time reference to one another and databases.
Conventional dates (i.e. "December 25, 1985 - 3:45pm") contain various characters that need to be converted to a useful format each time they are used. By storing timestamps as an integer (i.e. "504373500") it is much easier for a computer to store and work for comparison using basic math.
The Unix Epoch is the starting point created as a universal reference. All Unix Timestamps (sometimes also referred to as Unix time or POSIX time) are an integer that simply represent the number of seconds since that time (currently seconds since the Unix Epoch).