빠른 생일 (or 이른 생일) means "early birthday", i.e, the birth date is in the beginning months of the year. 늦은 생일 is the opposite. People are conscious of such things because certain age-related requirements may have different cut-off criteria which can sometimes give people with 빠른 생일 an advantage. A prime example is the school year. In Korea, grade schools start the school year on March 1st. Schools therefore admit children fully 6 years old on that day in 만 나이(the same biological age used internationally). This means for the 2016 school year, for example, only children born earlier than March 1 of 2010 are admitted. Therefore children born in January and February of 2010 are starting schooling at a younger age than the others born in the 12 month period of eligibility (2009/3/1 to 2010/2/28) - almost one year earlier in the extreme case. This head start can give them advantages later in life because they have effectively finished school sooner than the others. Korea being a fiercely competitive society, every little advantage counts, so people tend to be keen on even such a small accidental windfall.

I'm 빠른 생일 So I could explain with my example. Before we get to 빠른 생일, you need to understand 2 concepts. 1. Concept of hierarchy in age If someone is even one year older than you, then that person is not considered as your friend, but as 형/오빠/누나/언니 with higher rank. 2. Lunar Birthday You know the difference between solar date and lunar date right? In Korea, many people(mostly old people) follow lunar dates as their birthday. For example, I was born in 27th of Jan 1988. On that day, the Lunar date was 9th of December 1987. So my Lunar birthday is 9th of December 1987. Normally, there is about 1 or 2 months difference in days between Solar and lunar calendar. So, here is why 빠른 생일 is oftenly mentioned among people. In Korea, Kids start to go to school when they are the age of 8.(Korean age. That is one other thing you have to know. If you don't know what it means by Korean age, let me know, then i will explain you additionally.) I started to go to school in 1994 which is the year I am 7, and became friends with 8 years old kids. This was possible because of my Lunar birthday. Simply, I was considered as a 1987-born-kid because my lunar birthday is 1987. So I could have the same age heirarchy as 1987 kids, not 1988 kids. Koreans have to make some sort of agreement on how to call each other when they start relations, since we call and use the addressing names. easy example : If someone born in 1988 comes to me and say "We are the same age, so we are friends.(in age)" then I say. "But I am 빠른 생일. I was born in Jan."(implying that I went to school one year earlier and I have one year higher in aging hierarchy.) then the person will say. "Oh, I see. Then I should call you 형/오빠." This is how it works which will confuse you so much. Because of that sorts of confusions, nowadays, schools stopped to accept the 7 years old kids even if their lunar birth year is same as the 8 years old kids.

A 빠른 생일 is a 생일 (birthday) in the first two months (January & February).