Okay kiddo, imagine the ocean is like a big swimming pool. Subsea technology is like using special tools and machines to explore and do things in the deep parts of the pool where you can't touch the bottom.
Sometimes people need to go deeper in the ocean to do things like look for oil, gas, or minerals. Subsea technology helps them do that. They use special machines and robots that can go really deep underwater and bring back information or objects they find.
Subsea technology also helps people build things in the ocean like oil rigs or wind farms. They use special tools and machines that can work underwater to build and maintain these structures.
So basically, subsea technology helps us explore and do things in the deep parts of the ocean that we couldn't do without special tools and machines.