"Side activity" refers to any activity that is not primary activity. For example, a student has the primary activity of attending school. A "side activity" would be participation in some sort of singing group. A person's job would be their primary or essential activity. The hobby of electronics would be a "side activity". Any sort of recreation is a "side activity". Reading novels would be illustrative of a "side activity".

It means you should not put personal growth should be "put to the side" "ignored" "forgotten".