A panadapter connects to a radio and provides a "snapshot" view of signals on a given band. The display shown nearby illustrates what an operator might see. In this case, the display plots signal activity on a portion of the 40-M band centered at 7.220 MHz. The display shows "instantaneous" signal amplitude in the upper window and a "waterfall" display below. The waterfall shows activity over a given time. The brighter the color, the higher the amplitude of the signal. Based on the signals present you can get a good idea about band use and propagation.
Many hams can't afford an add-on panadapter, though. The unit shown here for an Elecraft transceiver can cost upwards of $US 700. Zowie! But hams still can see activity on many bands from many places on the globe. A worldwide view can help you determine which bands look open in a country or area you want to contact. Open a browser on your PC and check out the webSDR site. It gathers panadapter-type information from many software-defined radios (SDRs) and lets users select one and then choose a band to view. Not all SDR stations cover all bands, but you can get enough information from several to "scope out" band conditions and ham-radio activity.
The SDR Web user interface lets you move the receiver's "dial" to a signal of interest and listen to what goes on. Here's part of the display from F8KGO in France. The main WebSDR page lists 112 SDR stations and plots their locations on a small world map. --Jon Titus