Normal, point to point tickets are for a route. So with that ticket you can take a single trip on the route you bought, but you can choose which train you take, and can even break your journey. Only condition is that you finish the journey on the same day. So you can travel on other trains than the one you selected.
These tickets can also be refunded if you end up not using them (up to the day before they are valid).
Supersaver tickets are cheaper, but are only good for the train you selected, and cannot be refunded.
Normal point to point tickets are always available, and you can buy them any time, as long as you buy them before you board the train.
Supersaver ticekts you must buy in advance, and are in limited supply. When they’re gone, they’re gone.
For a trip to the airport a supersaver is a good idea, as you book your air travel far in advance as well, and this fixes your train travel. A supersaver is not good from the airport though, as you your flight might be late, causing you to miss your train…