Nature refers to certain qualities of the planets; some of them are considered Benefic, some Malefic and some Neutral.

Basic Nature of Planets are as under :-

Benefic planets are:
      Jupiter, Venus, Moon (waxing) and Mercury (well associated)

Malefic planets are: Sun, Mars, Saturn, Moon (waning) and Mercury (badly associated)   
Planets Nature will vary depending upon the Lagna at birth time.
For a particular horoscope the Table of Planetary positions indicates the nature of planets.