This PDF is a very good explanation of SWR. Club member Roger Steyaert K7RXV got permission from its author, John Fielding ZS5JF, to put a link to it on our web page.
http://zs6wr.co.za/documents/SWR.pdf
If you have issues accessing this document, ZS6WR is the club call for an amateur radio club in South Africa. You can use a Google search for ZS6WR and then on the right side of the home page click on antennas and you go to a list of antenna articles including this one.
From this right club website’s sidebar there are also links to lots of other educational resources you might find interesting: http://www.zs6wr.co.za/live/.
Remember that some of the terms used appear at first glance to be incorrect. You have to translate things into British English then everything is correct. Remember that South Africa was a Britsh Colony for many years.