Short term variable costs vary with the level of production – for example the more units are produced the more the total cost of materials will be.
Long term variable costs are fixed in the short-term but might change over time – for example the cost of maintaining the machines in the factory might be fixed in the short-term but if they improve their way of working and become more efficient then the cost might change in the longer term.
The distinction is unlikely to be relevant for Paper MA.