Mass is the amount of matter in an object. Its standard unit of measure is in kilograms (kg).
Weight is the force experienced by an object due to gravity.
In freefall or in zero-gravity, an object that has mass has no weight. An object on the Moon will have the same mass on Earth, although it will have 1/6th the weight.
For primary school it is usually the case that these can be used interchangeably, but bear this in mind in case of some tricky questions!