It means you make a mental note of the fact, or some other way to acknowledge it, whenever you see your birth date numbers. That makes it much more likely that you will remember those numbers long after you forgot all the other numbers you saw during the day.

Because you remember seeing those numbers and do not remember seeing the other numbers you saw, it feels like those are the only numbers you see.

See the Starting to See Certain Numbers Again and Again article for more information about that. And also Why You See Certain Numbers All the Time.