A piece of genealogical information provided by someone who had first hand knowledge of the event is said to be primary information. Other information is secondary. The closer the information is provided to when the event took place, the more reliable the information is considered to be.
My knowledge of my exact date of birth is not primary–I know it because I read it somewhere or I have been told it. My knowledge of my age, within a year or two, is primary. Having gone to school since the “age of five,” I have a good idea of just about how old I am based on my life experiences. I can provide primary information of when and where my children were born.
My knowledge of my father’s date and place of birth is secondary. I’ve seen it written in various places–official and otherwise–including his actual birth certificate. Just because my knowledge is secondary does not mean that it is incorrect. Classification of information as information as primary or secondary is merely stating how the individual came to know the information.