Retrograde means the planet appears to slow or move backwards from our viewpoint on Earth, sort of how a car passing another slower car in another lanes makes it appear that the slower car is going backwards. (Or a runner on the inside lane on a track can shotgun past the runner on the outside lane.) it is said that the effects of a retrograde can cause delays, turmoil or stagnation in the areas said planet rules. So we get Mercury Retrogrades about 3 times a year and Mercury rules communication and technology…so anticipate problems with tech, emails, computers on the fritz, car problems, etc.
With Pluto ruling transformation, secrets, Revolution, many astrologers are saying that mass secrets will be revealed during this time that will affect the global citizenry. (A “Great Awakening,” in other words.)
There is one exception to the rule that the third and final (direct) transit is the most important. For Mercury, it's the first direct transit, not the third, that is the most important.