Solunar Theory was conceived by John Alden Knight in 1926, and posits that the activity of fish and animals is influenced by the location of the moon. There are several times during the day called solunar periods or solunars when the activity of fish and animals is typically heightened:
Major Solunar Periods last approximately 2 hours, and occur when the moon is overhead, and when the moon is underfoot (on the opposite side of the planet). There are usually 2 Major Solunar periods per day, though this is not always the case.
Minor Solunar Periods last approximately 1 hour, and occur when the moon rises and when it sets. There are usually 2 Minor Solunar periods per day.
All the solunar periods are usually the best times to fish, and the major periods are typically even better than the minor periods.
Ultimately, there are many things that influence whether fish bite. The major factors are usually tides/tidal flow, the sun, and the moon, or solunar periods. This site lays out these major factors to help you find the best times to fish, and hopefully catch the most fish. On each prediction pages, you’ll see the times for each of the solunar periods laid out. The solunars are also represented on tidal charts as groups of fish, to help you compare the solunar periods to the tidal periods.
More reading can be done on Wikipedia’s Article on Solunar Theory