In many Moodle activities' description field there is an option to place a link to an external website. This can be useful if you'd like to share a resource with your students, but would like more attractive formatting. Google is more readable than https://google.com
In your new activity's description field, or elsewhere in Moodle, you'll see a link icon. Look for the chain link icon.
To make a link, highlight your text, then click the chain link icon.
After you click the chain link, you'll enter the link of the site you'd like.
I think the easiest way to get the link of a site is by navigating to the site. Once you've navigated to the site, click on the address in the address bar to highlight the whole URL, then right click -> Copy (you can also use keyboard shortcuts. CMD-C on Mac, or CTRL-C on Windows).
Back in Moodle, paste the link you've copied. If you'd like the link to open in a new window, select that checkbox.
Once you're ready to go, click "Create link".
You'll notice that the text you made into a link is now green.
You can continue the rest of your activity setup. There is no limit to the number of links you can place.