What is a Primary Atmosphere regarding a planet?
A primary atmosphere is the initial atmosphere a planet has shortly after it formed.
Earth has had a number of different atmospheres that have changed over time. Earth's first or primary atmosphere was probably made of of the same gases that were accumulating in the prorto-sun - hydrogen and helium, Perhaps also methane and ammonium.
After the Earth was hit by a stray proto-planet (which after collision with Earth became the moon), the initial atmosphere was probably blown away towards the larger Jovian planets. Energy bursts from the sun may have also stripped away Earth's primary atmosphere.
Over time the atmospheric composition changed a number of times and most recently (past 2 billion years) life itself added oxygen to the atmosphere.