What does "qualify for a more competitive rate" mean when booking flights for academic travel?

I know this question may be off-topic on academia SE, but really am not sure where to ask. I was admitted to a great PhD program recently, and was also invited to participate in their Open House in March. I was informed that I would be reimbursed for travel expenses. However the email says to book flights as early as possible to "qualify for a more competitive rate". Does this "rate" refer to the ticket price or the rate at which I am reimbursed? I'm afraid to ask the secretary since I don't want to make a bad impression.