If you order an aircraft it is for a firm delivery date and you will negotiate hard and get say a 737, whose "list" price (in practice rarely paid) is $60m each, for a bargain price, say $50m for each one, all in a firm contract
Airlines like to have one type in their fleet. Now I have bought 10 737s to be delivered over the next year at $50m each. But maybe in 3, maybe 5 years time I will want some more, though not really certain yet if I do or how many I will want, or when.
But if you go back to Boeing in a couple of years time and say "how much", they know you are stuck with all the maintenance and training for the 737 and that changing to the A320 will cost you a bomb and is not what you want to do at all. So they can say "Well the list price has now gone up to $70m, in fact they're so popular we would like $75m each".
So you do an option with your initial order that says - well they all say different things. But maybe it says you can order up to another 12, anytime up to 5 years from now, for delivery within 12 months from then, at a price of $50m plus some inflation index percentage. So when you do go back in a few years time you can be confident that the order price will be say $53m.
So you are on the order books with an agreed price although you needn't take any at all. But you know what you could have if you want, and how much they will cost.
If things turn out bad for the manufacturer there is of course nothing to stop you from negotiating the price down further at that later date. But the manufacturer cannot stick you for more.
Manufacturers like options too because they give an indication of what could possibly be sold in the future and the figures often get misinterpreted as something as good as orders for the long term.