1-line Product Positioning: The easiest way to enjoy online video and music on your TV
Product description: Chromecast is a thumb-sized media streaming device that plugs into the HDMI port of your TV. Set it up with a simple mobile app, then send your favorite online shows, movies, music and more to your TV using your smartphone, tablet or laptop.
I'm not finding "1-line product positioning" and that sentence next to each other. I do find "Chromecast: The easiest way to...". I wonder if you have stumbled across a pre-production version that has instructions meaning "Place Chromecast logo here on the same line."
What I can tell you from you post and from what I know positioning in marketing means how you position your product or service to the mind of your consumers or customers, that's what even marketers know, so from your thread, 1-line in 1-line product positioning can means,a line of products that you want to position to the mind of your customers, this is what marketers do before they launch their new product, now you can Google the meaning of a line of product(but I can guess maybe product that related but may differ in their features, say lipstick, powder ), hope help.