I feel dumb for asking, but I only just started playing Warframe again after being gone since Mesa Prime release.
Thermia Fractures are fractures that can be found throughout Vallis, and have to be sealed. Think of it like an open world Mobile Defense mission. You have to steal sealers from Coolant Raknoids in order to do this. I personally have issues with finding the fractures myself, but usually if you search for a game a couple times you'll find someone who knows what they're doing.