This is often called the main idea sentence, for it gives the topic plus the author's point-of-view on that topic.

It is the most abstract sentence, meaning it must be explained, shown, or proven by the rest of the sentences in order to be effective.

Ex. I typically hate the winter months here in Michigan, but going skiing on a regular basis has truly changed my negative perspective over the last few years.

In this sentence, the topic is "skiing" and the viewpoint about skiing is it "has truly changed my negative perspective."

Can I explain it further? Yes!