Imagine you are playing a game of tag with your friends. Soft martial arts is like tagging your friend gently with your hand. You might lightly touch them to show you could have hit them, but you don't want to hurt them.
Hard martial arts is more like accidentally running into your friend at full speed. You might not mean to hurt them, but you could accidentally hurt them pretty badly if you're not careful. It's more powerful and forceful, and can be dangerous if you're not paying attention.
Some martial arts focus on being soft, like Tai Chi or Aikido. Others focus on being hard, like Karate or Muay Thai. Both styles can be helpful in different situations, but it's important to be aware of the differences so you can use the right approach at the right time.