To flatten means to make something flat or to remove its thickness.
Imagine you have a big box of clothes that's all squished up and hard to carry. You might need to flatten it out so it's flat and easy to fold up and put away. This is what happens when you take the air out of an inflatable mattress too - it starts to flatten down. Sometimes we use machines to help us flatten things, like a big ironing press for thick blankets or clothes.
To make something flat or flatter.
Example: As there was a lot of damage, we chose the heavy roller to flatten the pitch.
