Beacons are radio transmitters that are set up on fixed frequencies for others to use as references. These beacons typically operate either full time or on a fixed, published schedule so you can count on their actions. Beacons can be as simple as a constant carrier, or they can be modulated with all kinds of tones/audio.
Listening to the same beacon, over a long period of time, you can learn a lot about propagation. The time of day, time of year, and time of solar cycle all play a great role in how those signals get from the beacon to your receiver. Take some time and listen for a few of the beacons listed on the following links.
North America Radio Beacons – LF/MF Radionavigation Stations
William Hepburn has compiled a great list of all North American beacons and selected world-wide beacons in the longwave (lw) and mediumwave (me) bands. This page lists beacons in the 195 kHz – 1737 kHz range. This list is available by clicking this link.