Empty a column
An empty column is the strongest thing you can own in FreeCell. It takes any card, so it is a free cell that also lets you unload a run onto it — and, unlike a free cell, it doubles the number of cards you can move in one go.
Why it doubles
The limit on a run is (1 + empty free cells) × 2^(empty columns). Each empty column multiplies, each free cell only adds. Four open cells and no empty column lets you move five cards; four open cells and one empty column lets you move ten.
What it costs
Emptying a column usually means breaking up a tidy sequence you built earlier, and beginners hate doing it. Do it anyway. A neat run of six cards that you cannot move is worth less than a hole you can put anything into.
Keeping it
Once you have one, do not fill it with the first king that comes along. Fill it when the card going in is one you need to move again soon, or when filling it unblocks something underneath.
On the board, this one reads: Empty that column. You will be able to move 10 cards at once instead of 5.