“A featured image represent the contents, mood, or theme of a post or page. Posts and pages can have a single featured image, which many themes and tools can use to enhance the presentation of your site.”
The feature image you choose for your article might just be the one thing that entices your visitor to click, read and find out more about what you have to say, your products or services, so choose carefully.
The official definition is actually pretty good. Your feature image should be representative of your content, tell a story if possible, or at least catch the viewers attention within the context of your content.
I always invite you to find out more about feature images by checking out the official wordpress documentation online!