It means when you see your birthday numbers you remember seeing them.

Out of all the numbers you see every day, those are the ones you remember. The reason you remember seeing them are two-fold:

1. The numbers mean something to you. They're your birth date. Not everyone remembers it when they see their birthdate numbers, but some people do. See Why You Start Seeing Certain Numbers Again and Again for an explanation.

2. You wonder or assume that seeing the number has a special personal meaning or message. With that question or assumption, it creates a mystery that feels like it needs to be solved. Therefore, you tend to remember seeing those numbers even more — because every time you see them you remember it, where before you might not have remembered every instance. See Why People See Certain Numbers All the Time for how that works.

You remember seeing the numbers because they mean something to you. And you keep on seeing the numbers again and again because you assume seeing the numbers means there's a special message for you.