You win when all 52 cards are on the foundations, each suit built from ace to king. Nearly every deal can be won, since every card is visible.
You win FreeCell by moving every card to the four foundations, so each suit runs from ace to king. The game is won the moment the last king goes home.
Can every deal be won?
Nearly all of them can. Every card is face up from the start, so careful planning matters more than luck, and a loss is usually a plan that did not look far enough ahead. Undo lets you back out of a dead end.
Getting better
Look at the whole layout before you move, free the aces and twos, and empty a column early. The FreeCell strategy guide sets this out, and how FreeCell is dealt and how the foundations work cover the basics. The full rules list everything in order.