Unisex means something that can be used by both boys and girls, or men and women. Let's say you have a toy that both boys and girls like to play with, it would be unisex. The term "uni" means "one", and "sex" means "gender", so unisex means something that is for only one gender.
For example, a shirt that is designed for both boys and girls to wear would be considered unisex because it can be worn by anyone, regardless of their gender. Unisex bathrooms are bathrooms that can be used by both boys and girls, and unisex haircuts are haircuts that can be worn by both boys and girls.
Unisex things are great because they make everyone feel included and they allow people to express themselves in their own unique ways without feeling pressured to conform to gender norms.