An arborist cares for the forests of civilized nations, keeping the trees free of dead growth and felling diseased ones. Planting new ones where needed, especially in previously felled areas, particularly important in areas of heavy lumberjacking.
Arborist don't spend a lot of time doing the hard labour, they use local lumberjacks to do that, they merely identify what needs to be done. Using cottars to plant new trees under their direct supervision.
While players are generally free to select a citizen role as a background, they should check with the Dungeon Master to make sure it fits within the local area, other party members and does not conflict with the intended adventure.
Citizens have proficiencies in weapons and armour relative to their type and region, so it is not included in the above information, and players do not gain any proficiencies from choosing a citizen background.