Okay kiddo, so a hydra in the Marvel Cinematic Universe (MCU) is a bad guy organization that first appeared in the movie Captain America: The First Avenger. They are a group of villains who are trying to take over the world and make it their own.
Hydra is led by a guy named Red Skull who is super evil and wants to dominate everything. They have a symbol that looks like a snake with a bunch of heads, which is where they get their name from (since the Hydra in Greek mythology was a serpent with multiple heads).
Hydra uses a lot of high-tech gadgets and weapons, like laser guns and flying ships, to try to defeat their enemies and take over. They even have their own version of the super-soldier serum (the thing that turned Captain America into a super strong hero), which they use to create their own soldiers and agents.
Hydra is really dangerous because they are basically a secret society. They have members all over the place, in different countries and different positions of power. So you never know who might be working for Hydra and trying to take over the world from the shadows.
But don't worry, the good guys like Captain America, Iron Man, and the other Avengers are always there to stop them and save the day. They fight Hydra and other bad guys to make sure that the world stays safe and free. So even though Hydra might seem really scary, the heroes of the MCU are always ready to stand up to them and protect us all.