A billboard consists of a very large (10 feet by 20 feet or larger) sign which carries a photograph or painted image that advertises a product or service. Billboards are often illuminated with lights and some may be back-lit or, in the 21st century may be huge flat-screen electronic displays.
Arguably, billboards started from two otherwise innocuous methods. The first was Burma-Shave's use of five small signs along roads giving a poem. The second was the practice of some companies agreeing to pay to paint a farmer's barn in heavily travelled areas in exchange for being allowed to paint their ad on the side of the barn that faced a major highway. Sometimes the farmer got additional payments in addition to the free paint job.
Billboards spread everywhere, especially along interstate highways. Billboards developed over the decades, with some upgrades being mechanically changeable billboards, which could display several images in a sequence, eith using rotating louvers or a conveyer-belt-style fabric panel on rollers. In the 21st century, billboards are now being revamped in the digital age. Now there are digital billboards that change images every few seconds. A good example of these are the huge digital billboards in New York's Times Square. This allows the same display to show different ads at different times and they can be changed for a much lower cost than old-fashioned painted billboards.
This is completely unrelated to Billboard magazine, which reports trends in the recording industry. It used to be related, however, which is why it's Billboard and not Record.