What does "you may run out of emails before the end of your billing period" mean?

What does "you may run out of emails before the end of your billing period" mean?

feedBAKER will keep track of how many emails you are sending daily and how many days you have left until your renewal date. This means that the system will warn you if it thinks that you're likely to run out of emails before your renewal date.
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Please note: If you do run out of emails before your renewal date you'll have a grace period of 24 hours to upgrade your plan before emails stop sending.