Parse means to go through the string one character at a time and convert each part of the string to a number. In the Date.parse function the string could look similar to this

Date.parse("Mon, 19 Nov 2001")

In the simplest terms, the computer goes through the string (i.e. parses) one character at a time and determines that 19 is the day, Nov means month 11 and 2001 the year – it will then be able to calculate the number of milliseconds for this date.