Light leaks happen when light starts to come out of the grow tent. They are bad because the whole point of using a grow tent is to make sure that all the light stays inside and is used for the growth and the development of the marijuana plant. However, if there are pinhole leaks taking place, this means that some of the light which should ideally be going to the plant is going out and is getting wasted. Hence it is critical to ensure there are not light leaks in your tent. If you spot any, try and fix them or let the company know.