Deductive reasoning is the process os using previously accepted statements to draw a conclusion.
I'll show you an example.
A detective determines that the person who committed the crime is a relative of the victim and walks with a limp. Since the mayor of the town is the only person who is a relative of the victim and walks with a limp, the detective uses deductive reasoning to determine that the mayor of the town must have committed the crime.
The previously accepted statements are in italics and the conclusion is shown in bold.