Remove Whitespace from Start and End: Question on Alternation Expression

JavaScript
1

In the " Remove Whitespace from Start and End" challenge, you must use the alternation expression | in order for the regex to work:
/^\s+|\s+$/g;

How come /^\s+\s+$/g; without the alternation expression not work? What is the alternation expression truly doing? How is it helping match the white space at the start and end?

2

This expression translates to “At the start of a line, one or more whitespace characters immediately followed by one or more whitespace characters and then the end of the line.”