Each classical planet (not includig Uranus, Neptune or Pluto) rules one or two signs of the zodiac. When a planet is in the sign of another’s rulership they are said to be received by them i.e. Sun in Aries is received by Mars. Mars is the landlord as he rules the sign and can be seen to be receiving a guest. Modern astrology only considers rulership by sign but traditional astrology also allows reception by exultation, triplicity, term and face. See Essential Dignity, Lee Lehmans web site or Deborah Houlding’s excellent SkyScript for further information and tables.
The example above is a one way reception. Mars receives the Sun. When both planets are in signs of each others rulership they are said to be in mutual reception i.e. Mercury in Taurus and Venus in Gemini. When two planets are in signs in which the other has some essential dignity they are in mixed reception i.e. Venus in terms of Jupiter, Jupiter in Venus’s triplicity.
Each planet is received by the ruler of the sign it is in. If it is in the sign of its rulership it can be viewed as receiving itself. Planets who receive each other will have a greater understanding of each others’ agenda than those who don’t.
A planets’ despositor and the despositor’s condition will have an influence on the condition of the original planet. For instance lets go back to the Sun in Aries. We need to look at Mars and this will be very different if Mars is in Capricorn sextile Jupiter in Pisces than if it is in Cancer conjunct Saturn.