I don't recall that this is specifically mentioned in this movie but in the comics Reed creates clothing from
A synthetic material created/discovered by the famous scientist Reed Richards, which can be altered easily and adapt to a certain environment, allowing them to be incredibly resilient to drastic changes in heat, cold, pressure, density, dirt, etc. making them ideal for use as superhero costumes.
Unstable Molecules are configuration of unknown atomic nuclei and electrons discovered or synthesized by Reed Richards, which is responsive to certain energized matter around it. Used in the making of clothing for the Fantastic Four and certain other superhuman beings, unstable molecular cloth mimics its wearer's physical properties, it can turns invisible like The Invisible Woman, allowing it to stretch like Mister Fantastic and burst into flame without burning like the Human Torch.
As it happens, this is mentioned in the first Fantastic Four movie script...
Material made from self-regulating
unstable molecules. I've been working
on a formula for this.