Dōjin soft is like when you and your friends make up your own stories or games and create them together. It's when people who love a certain thing, like video games or anime, create their own stories, games, or artwork inspired by that thing.
Imagine you really love Minecraft and you and your friends create a new world with different creatures and rules. That's kind of like what dōjin soft is.
Sometimes people share their dōjin soft with others, and sometimes they even sell it, but it's not like the big companies that make the original video games or anime. It's like a fun hobby that people do because they love something so much they want to create their own things inspired by it.