Keep Page URL fragments when tracking Page URL

General questions
1

I like to know what is definition for the following settings:
Keep Page URL fragments when tracking Page URL
No/Yes

If we set no, will this URL eliminate www.mydomain.com and keep en-GB?

2

Not quite.
URL Fragments are part of hypertext.

In www.example.org/foo.html#bar, the last three letters on the right, bar, is the fragment Hypertext sees the part after the pound sign as an element with id="bar".

So in Matomo settings, you can toggle keeping/discarding this little part on the right-side of the URL
Matomo >> :gear: >> Websites/Measurables >> Manage >> “Keep Page URL fragments when tracking”

More on fragments and hypertext at Wikipedia