A "Primary Record" is generally considered to be a record of an event that was recorded at or soon after the time of the event by parties directly involved in it. A certified copy of a birth certificate filed at the time of birth would be a good example of a primary record.
A "Secondary Record" is, on the other hand, a record of a more dubious quality. It may or may not be accurate, but even if it is accurate, it is not considered as "proof". For instance, I could tell you that I was born on August 18, 1959. But that is not a primary piece of evidence. I was obviously present at my own birth, but just as obviously I have no direct memory of it. I could be lying for some reason. Or I could be sincere, but wrong, if my parents gave me bad information for example. A good researcher will search for other records to substantiate my claim.
A very good example of an incorrect secondary record is the date/place of birth for my mother Lois Rebecca Canfield, as given on her death certificate. The informant [my sister Cindi] had the date correct, but the place of birth was WAY off, not only the wrong county, but even the wrong state. A genealogist that accepted Mom's death certificate information as real proof of her birth information would be not only out of luck, but foolish. Fortunately, we have a copy of Mom's birth certificate, so no genealogical "harm" was done.