A Reducing Atmosphere lacks oxygen gas or other oxidizing gases (i.e., lacks gases with a high Oxidation State). Unlike Earth's Atmosphere, such an atmosphere does not enable oxidation of substances in contact with the atmosphere. It constitutes non-oxidizing gases, potentially including reducing gases.
Planets can have reducing atmospheres, and early Earth is theorized to have had such an atmosphere before biological processes produced oxygen. Chemical processes in reducing atmospheres are a potential source of complex organic chemicals, and the theorized atmosphere could have been a factor in the development of life. Artificially-created reducing atmospheres are used in some Metal processing such as Annealing.
An Oxidation Reduction Reaction or Redox Reaction is a chemical reaction where one chemical is Oxidized, i.e., loses electrons, and another is Reduced, i.e., gains the electrons. A chemical that tends to lose electrons (i.e., itself undergoes oxidation and encourages the reduction of another chemical) is called a Reducing Agent, and if it is a gas, a Reducing Gas. Conversely, a chemical that tends to gain electrons (i.e., itself undergoes reduction and encourages the oxidation of another chemical) is a Oxidizing Agent (e.g., Oxygen), and if a gas, an Oxidizing Gas.
The term Oxidation was first used to name what happens when oxygen reacts to other chemicals. The term Reduction stems from its adoption to describe the extraction of metal from ores. A reducing agent with a very strong "tendency to reduce" is used in a chemical reaction that first "un-reduces" the metal oxide's oxygen (leaving the metal itself) so the oxygen and reducing agent can carry out their Redox reaction. The "reduction" is the elimination of oxygen from the metal oxide within the ore.
Gas mixtures classified as "reducing atmospheres", by definition, can include reducing gases such as Hydrogen gas and/or gases that are neither reducing nor oxidizing agents, such as Carbon Dioxide. If an atmosphere were to have both reducing and oxidizing gases, it would be less stable, and inclined to eliminate one through chemical reaction.