Trance came out of the 1990s as a melodic rhythm set with synthesizers that has the distinction of repetition throughout the song. Only a few minute changes occur throughout the song, such as a differing drum beat or synthesizer bridge.
Trance patches are small pieces of a trance song, typically sold in batches or groups, that can be incorporated into a producer's or DJ's original song piece. Since much music is now produced on computers, these patches can be downloaded and altered by music software to produce new and exotic sounds.
Though the trance patches are copyrighted by their respective creator, most purchases come with an agreement. The purchase effectively buys rights to utilize the sounds without any need to pay royalties to the creator if the resulting music makes money.