"Know Thyself" is a phrase that means understanding yourself. Imagine that you are a very important scientist and you are studying a new thing which is called "You".
It's like learning about a new animal, but this animal is you! You have to ask yourself questions like: "What makes me happy?" "What makes me sad?" "What do I like to do?" "What do I not like to do?" "What am I good at?" "What am I not good at?"
You might feel like you already know these things about yourself, but it's important to really think and learn about yourself so that you can make choices and decisions that make you happy and successful.
It's like when you're trying to do a puzzle, you need to look at each piece very closely to understand where it fits in the puzzle. In the same way, when you "know thyself", you understand yourself so well that you can fit in the world around you and feel good about it. So take the time to think about who you are, what you like, and how you can be your best self!