Fuuinjutsu entails the process of sealing. This is basically an art, or method, to seal chakra, weapons, intangible elements like Fire and Lightning, inside a seal. Basically, these things, are taken into the seal, despite their size or whatever, and "hang" in another "realm" for the lack of a better word. Not only can things be sealed into seals, but also be brought out of them like weapons for example. I've seen some seals, that if triggered by a specific stimuli, activate and then automatically release whatever is inside it. However, things can also be sealed into some other things, like tattoos, or even more solid like objects, like weapons, I think, Overall, the premise is very simple, sealing of things, and also bringing things from where they were sealed in order to use it in battle. Some seals can be used to contain chakra, limit an opponent's use of chakra, like that seal used on Naruto by Orochimaru. It has a bunch of uses overall. I've seen seals that are able to release people from Genjutsu, release and maintain chakra, and even limit people to certain elements (shoutout Zanda).