Mass specifically is a measure of weight and volume in space and represents a measure of inertia. more mass generally constitutes larger or more densely packed items. Any ship or cargo within the game has mass. Megacredits have a negligible mass.
The larger a ship's mass, the more Fuel is required to move the ship (increasing the cost of acceleration and deceleration).
Kilotons are the units of measure for mass. More kilotons is more mass. Anything with a weight in kilotons is Mass. Kiloton is often abbreviated with KT or kt