Say you have two folders, parent, and child. Child is a subfolder of parent. You have a file in parent called 'xxx.html', and one in child called 'yyy.html'. So, your structure would look like this:
Now, imagine the user has browsed to yyy.html. If you want to link to xxx.html in yyy.html, then you'll need this relative path:
You can imagine relative paths as a cursor. '../' means 'Take the cursor to the parent folder'. So, a relative path, '../xxx.html' means 'Take the cursor to the parent folder, and look for a file called "xxx.html"'. Here's some more complicated examples:
will take you to c1page.html from c2c1page.html (nb: it will also work if you place the relative path in c2c2page.html)
will take you from c1page.html to c2c2page.html. This path could be shorter, but that's not the point.