Neighborhoods are the areas that you can visit from a lot without going to the world-view, just by walking over and clicking the visible house/business/lot in the live game mode. They are shown in world view with little colorful icons that give a name and brief description of that area when you mouse over them.
So, for example, in this image of Henford-on-Bagley, the neighborhood names have been filled in. It has 3 neighborhoods: Bramblewood, Old New Henford and Finchwick. All 4 lots in Finchwick will have the same initiatives and eco footprint, but the homes in Bramblewood and Old New Henford would have different ones.
https://i.imgur.com/ei1qR2x.jpg
When you are in the world view there's a button at the top to toggle the display of eco footprints. When you do that, the individual neighborhoods are demarcated with circles (in the eco footprint color) drawn around each neighborhood. So like in this image you can see 3 different neighborhoods circled, each which has their own eco footprint, but ALL lots in each of those circles is the same.
https://www.carls-sims-4-guide.com/gamepictures/expansionpacks/ecolifestyle/ecofootprint/how-eco-footprint.jpg