Hot spot tape is used to help prolong the life of the plastic cover.
The metal structure of a polytunnel will heat up in the sunlight more than the surrounding air. This means that the plastic sheet that is in direct contact with the framework will become hotter than the area of the sheet not in contact. As a result that part of the sheet can start to breakdown faster.
To prevent this, Hot Spot tape is used, for example, on the outer side of each metal hoop, so that the plastic doesn't come into direct contact with the hoop.