There are four free cells, and each holds one card of any kind. Park a card to unblock a column, then move it out again when a spot opens.
The four free cells are temporary parking. Each one holds a single card, any card, and a card in a cell can later go onto a column or a foundation when it fits.
What they cannot do
- A cell holds one card only, so you cannot park a whole sequence in one.
- A full cell stays blocked until you move its card out.
Why to keep them empty
Every empty free cell lets you move one more card at once (how many cards you can move at once). Filling all four early is the quickest way to get stuck, so use a cell only to free something specific, such as an ace. The FreeCell strategy guide goes through this, and the full rules cover everything else.