Oxidation is a term for a specific type of chemical reaction. If a chemical is oxidized it means that it loses some of its electrons. An oxidizing agent is the chemical species in a chemical reaction that causes the material to be oxidized. In other words, the oxidizing agent accepts the lost electrons from the oxidized chemical. In other words, the oxidizing agent gets reduced to allow the oxidation to take place.
One of the most common oxidizing agents is molecular gaseous oxygen O2. The burning of methane is an example of an oxidation reaction.
CH4 + 2O2 --> CO2 + 2H2O
The carbon in the methane (CH4) is oxidized. The carbon in methane has an oxidation number of -4. The carbon in CO2 has an oxidation number of +4. So that is a loss of 8 electrons. The 8 electrons are accepted by the oxidizing agent O2, which as a result gets reduced.