Answer 1

Here's a short explanation and reference iink:
https://www.sololearn.com/discuss/974060/?ref=app

Answer 2

After you have mentioned a base as mentioned by Kirk Schafer all of your url's will be worked relative to the base url. For example you have named the base as https://www.sololearn.com you can refer to its discuss folder only by using /discuss instead of writing https://www.sololearn.com/discuss

Answer 3

There's a great explanation here:
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/base