You need a hose to hook up the city water to your RV - a specific type of hose that doesn't have lead or BPAs, since you'll be drinking from it.
You obviously want your hose to be long enough to reach from the spigot to your RV. 25' is typically long enough.
If you're boondocking, a longer hose can be a convenience if the fresh water spigot is relatively far from the dump station, but you can get by with a short one (you'll just sometimes have to move to get closer after dumping)
If you're camping somewhere that gets below freezing and want to stay hooked up to water, you might want a heated hose. This will prevent the water in the hose from freezing and bursting the hose. Heated hoses plug into a 120V receptacle and keep the water inside warm enough that it won't freeze. They are however much more expensive