At a busy airport, an airline cannot simply decide to arrive at eight in the morning. The right to use the runway at a particular time is allocated in advance, and it is genuinely scarce.
The runway sets a hard ceiling
A runway can accept a limited number of movements per hour, governed by the separation aircraft need behind one another and by the time each takes to clear.
Terminal stands, security capacity and, at many airports, noise restrictions on night operations narrow that figure further.
Where demand exceeds this ceiling, the airport is declared coordinated and every arrival and departure must hold an allocated slot before it can be scheduled.
Allocation runs on historic precedent
Slots are assigned for a scheduling season, and an airline that used a slot sufficiently in the equivalent season keeps it for the next one.
That continuity is what allows airlines to sell tickets and plan aircraft rotations a year ahead, since a route cannot be built on a landing time that might vanish.
It also means that at the most constrained airports, almost every attractive slot is already held, and the pool available to a newcomer is close to empty.
Use it or lose it
A carrier must operate a slot for a set proportion of the season or forfeit it back into the pool for reallocation.
Without that rule, an airline could hold slots purely to keep competitors out, and the airport's scarce capacity would sit unused.
The rule can produce perverse outcomes, notably flights operated at very low load factors purely to protect a valuable time, and administrators have periodically suspended it during severe demand shocks.
Slots behave like assets
Because a peak-time slot at a major hub is effectively a permanent licence to serve a lucrative market, it carries substantial value in its own right.
At some airports slots may be traded or exchanged between carriers, and they routinely appear as a material consideration in airline mergers and acquisitions.
Competition authorities frequently require slots to be released to rivals as a condition of approving such deals, precisely because holding them is what blocks entry.
What this means for passengers
Odd departure times often reflect the only slot an airline could obtain rather than any judgement about when travellers would like to fly.
Routes to congested airports are also thinner and more expensive than the underlying demand would suggest, because capacity cannot expand to meet it.
This is why building a new runway changes fares on a whole network of routes rather than only at the airport where the concrete is poured.