Hey there! I'm happy to help!
Density is how much mass there is per amount of space. The formula for it is mass divided by the volume. How much mass is there per (per means divide) amount of space?
Think of it like a room in a house. If the room only has one person, it isn't going to be very dense with people because there is only one person in the entire room. If there were 100 people, it would be very crowded in that room and the whole room would weigh a lot more. If we make our room the size of a huge convention hall though, 100 people wouldn't make it very crowded,. and it would be less dense. So the amount of people (mass) and the size of the room (volume) affect density.
In that sense, density is kind of like how crowded a thing is.
A hollow watermelon has less density than one that isn't because a watermelon that isn't hollow has more substance inside of it (more mass).
Mass is a factor of density because it measures how much of something there is. It can be related to the people in the room.
Weight is mass but with gravitational influence. A watermelon in space is going to weigh less than a watermelon on Earth because in space there is no gravity and it is floating around, but it will still have the same density just like our room full of people would still be crowded even if the people weighed nothing and were floating around.
On Earth though, we measure mass with terms like pounds, kilograms, etc. We use the term mass over weight because it is more scientifically accurate when calculating density.
Have a wonderful day and keep on learning! :D
