What are Band Conditions Banners?
Band conditions banners are graphical displays on a webpage, designed to present online a concise, easy-to-understand information about the current or predicted radio propagation conditions, for specific ham bands. Online banners display the current or predicted status of band conditions - whether a band is open or closed - as well as additional useful information. It is based on real-time data gathered from a variety of sources.
See an example with explanations of a banner showing ham band conditions (courtesy of Paul L Herrman, N0NBH):
The project "Understanding HF Propagation" provides a detailed overview and tutorials on HF propagation.