TRAILS is a solitaire card game. All the cards are dealt face-up into four rows of thirteen. The aces are removed, leaving four empty slots. The object of the game is to re-arrange the cards such that each row starts with a two, and ends with the king of the same suit. The row would read from two through king, all of the same suit.
To move a card, click on it. The only card that can be moved to an empty slot would be the card that follows (in order and suit) the card to the left of the empty slot. For example, if an empty slot has a 4 of hearts to its left, only the 5 of hearts can be moved to that empty slot.

For detailed rules, use the Help-->Rules... menu from within the application.