"Antenna lines" refers to the visible lines or bands on the back of electronic devices, such as smartphones, that are used to transmit and receive signals, like Wi-Fi or cellular connectivity. Full definition
Toss in a couple of antenna lines on the top and bottom and a fingerprint sensor dead center of the back, and you've got a fairly generic looking phone. (androidcentral.com)
The aluminum rear is polished, rather shiny, and continuous, with antenna lines located at the top and bottom. (9to5google.com)
The curved antenna lines at the top and bottom of the phone are barely noticeable. (androidauthority.com)