I got stuck when reading Differential topology 46 years later in the last section of the article ("Further details"). It is a summary of what is known about stable homotopy groups of spheres $\Pi_n$. It is stated that "the most difficult part is the $2$-primary component" and also $p$-primary components are mentioned.
What is a "$p$-primary component" and what does it have to do with a homotopy group of spheres? Recently I started reading different papers and articles about differential topology but it is the first time that I encounter this concept but it is not explained in the article.