What is “default language”?

Hi!
This may be a simple question: What exactly is default language and what happens if you change it?

My site has Swedish as default language right now (and no browser detection activated) so if a visitor goes to the website they come to https://www.mydomain.se. If have also setup English (www.mydomain.se/en/) and German (www.mydomain.se/de/). If I change my default language to English would then English be shown at https://www.mydomain.se or at https://www.mydomain.se/en/?

I am asking since when I tested nothing happened (except https://www.mydomain.se/en/ showing a 404.)

Kind regards,
Jesper