These sites use JavaScript to disable the copying of text, as well as other features such as right-clicking. Chrome has several addons that can toggle JavaScript on and off. Firefox does as well.
The first thing to try is viewing the page source, i.e. the code that makes the site. This may or may not show you the text you want to copy, due to how JavaScript works. Different browsers use different methods to view the page source, but that is easy to google an answer for.
Disabling JavaScript may or may not work for you though. Disabling JavaScript will require reloading the page. This may result in the page not functioning properly and not displaying the text you want to copy.
A text based browser like Links, or Brow.Sh may also work, as they support JavaScript. So you may be able to capture the text output with a redirection.