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).
Fuinjutsu is the art of sealing things (captain obvious entered the building) with a specific vase or material. The seal is triggered, either by the user, be used as a trap, or over a specific set of time. kanji or hand seals are usually used for the purpose to bind and continually hold the material in place, while chakra is used to maintain it over a duration depending on the type you use, Kanji would be something dormant and self sealing, while hand seal is something you have to focus on yourself, like a streaming technique. The seal can be broken through specific stuff like keys, by the users commands, or in some cases, but isn't seen much, through an amazingly strong force to break the seal. Just about anything can be sealed, that includes people, energy/chakra, material, tools, weapons, etc. Some people are more adapt to sealing, while others not as much, Uzumaki were well known for their sealing abilities. You also have people like TenTen who constantly seal weapons and release them. Some more advance, others more basic. Sealing can be the different of life and death, and they can be so powerful even the tailed beasts can succumb to their power. Some seals need more people to be created as well. Fuinjutsu is an advance extension of ninjutsu, and some medical ninjas have even more power/knowledge with the ability. Also, a lot of Kinjutsu revolve around fuinjutsu. It is truly a versatile ability.
A seal itself is like a dish with 6 main ingredients that come together to form it. The main object which is being sealed, the vessel it is sealed in, a trigger which initiates the seal, the object used to bind the target, chakra used to fuel the seal, and finally a key to release the seal. It sounds more complex than it really is. In practice, most of these are relatively simple.
Then, we move onto practical applications of seals. By sealing weapons, they can be more easily and quickly accessed during battle. For example, it'd be simpler to tap my wrist to summon a sword, than to draw it from my side. This can add an element of surprise to your battles, making it difficult for someone to react. You can also seal bodies, for easy storage after killing an opponent. You can seal incoming techniques, you can seal explosions and release them as the opponent nears, you can seal skills of the opponent, his chakra, limbs, etc. The possibilities are almost endless.