Tent caterpillars, they usually seem pretty cool when you first see them in the woods as a kid. However, when you get older and have trees on your own property they can quickly come across as unwanted guests.
Normally, when you see one here and there, they are pretty harmless. It is when you get large infestations that you see your trees quickly losing their leaves.
Control can be as simple as the physical removal of the tents. There are also plenty of biological sprays available. It’s best to shy away from synthetic pesticides due to undesirable side-effects.