nextval parameter is not clear

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/14/functions-sequence.html
Description:

https://www.postgresql.org/docs/14/functions-sequence.html

I don't see here any hints about how to use this function, and what the
regclass means.
How should I understand that sequence_name should be passed as string in
nextval('sequence_name') ?

Maybe add some links to other topics or clarify examples for nextval here.

Thanks.