This phrase is used by astrologers, but in the end it is often a value judgment. See below. A powerful chart can take many forms. One is to have many planets in your dominion or exaltation and those planets ruling the angles and/or in them, or some of them. Another way is to have these planets very well positioned, say in the MC or ASC.
A powerful planet is a planet that is in a good sign (for that planet), above the horizon, moving straight ahead, fast (i.e. traveling faster than its average speed in its orbit ), in a good or angular house, not applying an unfavorable aspect to evil ones and applying a favorable aspect to beneficent ones. It may be impossible for a planet to have all favorable conditions on a map (or unfavorable conditions), but having some of them makes a planet powerful.
That said, I will quote two charts that make us pass Hmmmmm. General George S Patton has a bad picture in every way, and he has achieved a lot in his life. He had a lot to overcome. At the other extreme I have a graphic in my files of a man who has almost every planet in great dignity and let’s be nice and say he hasn’t done much with his life until here. Strength or weakness shows how difficult or easy things can be, but it is up to the individual to overcome or set a direction and work with the chart to have a successful life.