Oxidation is loss of electrons. Reduction is gain of electrons.
An oxidising agent takes in electrons. A reducing agent gives them out.
Both ozone and chlorine are powerful oxidising agents.
Iron(II) choride can lose electrons to form iron(III):
A solution of iron(II) chloride will slowly oxidise to iron(III) in the presence of air.
Sodium sulfite is also a reducing agent:
It is used as an anti - oxidant in food preservation.