What is the Year Zero, Year One, Earth One, Earth Zero? How does this relate, if at all, to the DC Universe Rebirth?

Should I go back and read the characters from New 52 into and past the rebirth? I think this is somehow related to the different number of Earths and Years. Would I be better served finding an anthology a la something like this: The Flash: A Celebration of 75 years https://www.amazon.com/dp/1401251781/ref=cm_sw_r_cp_apa_Ux3ZBb62NHG28?

I'm just going through some main characters and reading everything starting at Rebirth. I gather there's not a particular order, really?

Also, why is Justice League volume 6 a different run and story and not a continuation from vol 5?