what is a “secondary dimension” in google analytics?

It is a way to filter your results by setting the filtering parameter. For example you can have the acquision channel as you primary dimension- that will show you how do you acquire users. And then, the country as the secondary parameter, so that the per channel results will be categorized according to the users' country.

Secondary dimensions are a way to see an additional breakdown of data. For example you may be viewing data based on Page Title as the primary dimension, but then select "Page" as the secondary dimension so you can see what specific URLs were driving the traffic for that same Page Title.