chop is workers chopping down forests. you can only do it after bronzeworking.
backfill, i use it to mean something like: ok, here's my official cultural territory, then there's land area behind me, but no competitors are there. i want that land (i already think of it as mine haha), but claiming it isn't a huge priority early on, since the AI will tend to expand closer to itself and not go through me to go behind me, so far from their homebase, early in the game. so, i expand towards them (either by settlers or capturing their cities) and do the "backfilling" as i have the time. also, cities farther from my capital are easier on my finances a bit later, with courthouses and such, early i tend to settle close cities, with exceptions for capturing juicy capital locations the other guy doesn't deserve to keep.
this changes. the AI does get into permanent "more cities! more cities!" mode. they always have a settler/defender on a boat looking for any space in the late game, no matter how crappy. earlier than that, when they have galleons and their own "core empire" i guess near their capital, then they will try to grab land behind you. so i don't put my backfill phase off too long, and i do try to grab the best coastal spots first; those will be the ones the AI sees first. depending on wars/relations/how far away the guy is i either don't open borders until i make the main cities i want, or i watch out for his settlers and close them at any sign.