seed titles, F1?

okay so i have been seed shopping online and i had some questions about what the numbers and titles meant, if they mean anything.

first when i was reading a few sites they said that these were F1, or F2 etc. seeds. does that mean they are from clones? does the F1 mean anything particular?

and when the site tells you how much you would yield per sq meter. are they implying clones or sea of green?

this is about the F1 and F2 found in the growfaqs, a great place for info.

https://www.rollitup.org/view.php?pg=faq&cmd=article&id=238

yield/sqm listed is grown under the best conditions with everything perfect and a veg. of about 6 weeks plus.