AddOns (sometimes called addons, add-ons, or mods) are collections of Lua/XML code that augment, modify, or replace World of Warcraft's default user interface. Addons vary greatly in scope and size — they may display information not presented at all by the default UI, add functionality to the default UI (e.g. additional buttons you can press to do things), modify the default UI's appearance (e.g. by changing colors or positions of UI elements) without changing its functionality, or even replace the default UI entirely.
As for the classes themselves, they almost all play like slower and slightly less interesting versions of their Legion counterparts. This is thanks in large part to the loss of one ability from each class that came along with Legion’s own AP dumping system: the artifact weapon, which added a unique ability to every spec in the game. The loss of these abilities left most classes feeling frustrating and incomplete, and though Azerite gear was supposed to be the replacement, it missed the mark completely. There will always be one Azerite trait that reigns supreme for each spec, meaning that the frustration of that particular loop will be unavoidable as long as the system is in the game.
Special note: At 110 it's almost certainly worthwhile to go and immediately get your Heart of Azeroth and first piece of Azerite gear from the BfA opening quest. It doesn't take long and provides pretty high power levels. I'd then return to Legion quests until 111 or even 112, since the power of these items will increase your kill speed significantly over the scaled enemies of BfA content.