The mass of an object is proportional to the amount of matter that it contains (matter is the substance of which things are made). The more the mass the harder it is to move. A feather's mass is lower than that of a brick, so it is harder to move a brick than a feather. Mass is measured in kilograms and an object's mass always remains the same. The larger the mass of an object, the more that other objects are attracted to it. The sun's mass is larger than that of the Earth's, so the sun attracts the Earth. The force by which objects are attracted to each other is called gravity.
The weight of an object (unlike the mass) will change depending on the force of gravity attracting it to another object. The weight (measured in newtons) therefore depends on the mass of the object. The moon has less gravity than the Earth so an object on the moon will weigh less than it does on Earth. Confusingly, in everyday conversation weight and mass are mixed up. When people say how much does it 'weigh' people often mean the mass.