What does interference mean in a engine?

It means that a fully open valve in the engine occupies the same space as a piston at the top of its travel. Of course under normal conditions this never happens at the same time.

When a timing belt breaks the valves freeze at whatever position they are at the moment of the belt failure. As the rest of the engine coasts to a stop if a valve is fully open it will be contacted by the piston and do great amounts of harm.

In a non-interference engine the two moving parts have no potential to contact each other. When a timing belt breaks the engine can be repaired by replacing the timing belt and retiming the engine.

Don't take the chance. Replace the belt and water pump when the manual says to. The belt could break just as you are trying to beat that semi...