It means that there is only 1 inventory in the booking class that that fare books into. This can easily be checked on a site like expertflyer. Once that inventory is used, subsequent tickets will result in a higher booking class, which will have a higher fare.

Of course inventory can also increase, for example if someone cancels or changes a flight, or if the airline decides to release more. But this is relatively rare (relative to people booking flights, causing inventory to decrease). So inventory does tend to decrease (rather than increase) over time.

It's definitely not just marketing.