"Bring your own device" means that you get to use your own phone or tablet for school or work instead of using a device that belongs to the school or office. It's like bringing your own toy to play with instead of using one that belongs to someone else.
When you bring your device, you have to be careful to make sure it doesn't have any games or apps that aren't allowed for school or work. It's like cleaning up your toys before you bring them to someone else's house to play.
Sometimes, schools and offices will help you set up your device so you can use it for what you need. It's like when your parents help you put together a puzzle or game so you can play with it.
Overall, bringing your own device helps you be more comfortable and familiar with what you're using. It's like wearing your favorite shirt to school or bringing your favorite stuffed animal to bed.