Field Effects are a battle mechanic introduced in Pokémon Reborn.
Though previous games incorporated the environment of a battle in some aspects, such as in the moves Camouflage and Nature Power, as well as the more recent Terrain moves and Natural Objects, Field Effects take this concept much further, allowing for much more use and strategies.
There are currently 30 released Field Effects, with plans to add five more in Episode 16. Most Field Effects can be summoned by certain moves or combinations of moves, but a number require a certain location. A select few are essentially unalterable, as Field Effects summoned by moves often dissipate after a certain number of turns.
The full effects of each individual Field Effect can be found in the Beryl Mansion, or on their individual pages. Prior to a large update on the official site, there was a section that listed all field effects and their properties.