Genealogists typically refer to information as primary if it is given by someone who had firsthand knowledge of the event to which the information refers. Other information is said to be secondary.
I was present at my wedding. I have first hand knowledge of that. I was present at my daughter’s birth. I have first hand knowledge of that information. I was present at my birth, but I do not have first hand knowledge of the date I was born. Similarly my own knowledge of my grandparents’ dates of marriage is secondary–I was not there.
Classification of information as primary or secondary is simply about how the knowledge was acquired by the person sharing it. How reliable they are is another issue–partially related, but also dependent on the information provider’s memory, the time that has elapsed since the event occurred, the person’s propensity to lie, etc.