VTOL Hover is a utility ability available for the Northstar chassis, including the Northstar Prime and Brute load-outs. It allows the user to hover for a short time, enabling them to see over buildings for sniping and scouting purposes. However, using this will make you more vulnerable, as you are becoming more noticeable and you move slower, making you an easy target.
Even though the VTOL Hover will allow Northstar to ascend above certain obstacles on maps, it is impossible to actually walk on the roofs of buildings - this is a balance feature. Despite this, some areas such as the "pier" on Drydock can be boarded.