Retrograde planets
A retrograde planet moves backwards through the zodiac for a few weeks, as seen from the Earth. It is worth being clear first about what is actually going on.
What retrograde is astronomically
No planet travels backwards. Retrogradation is an effect of the line of sight: the Earth and the planet both orbit the Sun, and when the Earth overtakes an outer planet, that planet appears to fall back against the background of stars — much as a slower train, watched from the window, seems to be moving backwards.
Mercury is retrograde three or four times a year for about three weeks, Venus roughly every eighteen months, Mars every two years and a bit. The outer planets are retrograde for a considerable part of the year — Saturn for some four and a half months.
That last point matters for keeping this in proportion: a retrograde Saturn in a birth chart is nothing rare. About one person in three has one.
How the tradition reads it
As a weakening of accidental dignity. A retrograde planet cannot carry out in a straight line what it stands for: it takes back, it repeats, it goes over the same ground again.
What matters is that this is a statement about execution, not about meaning. A retrograde Jupiter still means growth and generosity — they simply arrive by roundabout ways.
Retrogradation stands alongside other weakenings here, and it weighs no more than they do: combustion weighs more, and so does a position in fall. A retrograde planet in its own sign is better off than a direct one in detriment.
In the birth chart
Since a third of all people have a retrograde Saturn, retrogradation on its own is no ground for singling anything out. It is read as: this area of life takes several attempts.
Which area that is follows from the houses the planet rules — not from general statements about the planet.
On “Mercury retrograde”
The most popular astrological claim there is, and the most poorly grounded.
Mercury is retrograde for about eighteen per cent of the year. Whatever happens in those weeks in the way of misunderstandings, delays and technical breakdowns happens just as much in the other eighty-two per cent — it is simply not put down to anything there. This is a classic case of selective perception, and it can be tested both ways by keeping a diary of mishaps without looking up where Mercury stands.
The traditional use is narrower and more durable: in horary astrology a retrograde significator is an indication that a matter is coming back or being undone. That is a statement about a chart, not about a stretch of time in which everyone is supposed to go through the same thing.
See also: the planets and their condition.