Great Question! Mass is basically how heavy Something is. Most people confuse mass with weight but there very different from each other. Weight is the downward force something has because of gravity. Mass can be measured in a variety of units too. The second part of the question is how it affects density. It affects it because to find the density you need to be able to find the mass since the formula is Mass divided by volume. I hope this helped you understand a little bit more about mass. Have a great day!