Gemstones left outside during a highstorm become infused with Stormlight. The Stormlight can then be used to fuel the operation of fabrials or Surgebinding abilities. When Stormlight is not encased within a gemstone, such as when held by a Surgebinder or leaking from damaged Shardplate, it appears as a luminescent white vapor. It also increases the speed at which plants grow, hinting at a possible link to Cultivation.
Nightblood, the sentient sword from Nalthis, the location of the novel Warbreaker, is powered through the use of Breath. Brandon has hinted that Nightblood can also be harnessed through the use of Stormlight, leading us to assume that Breath and Stormlight are similar, if not exactly the same.