Most of the games have the choice to be 'Guaranteed Solvable' if you wish!
This means that the game dealt is possible to win - however it doesn't mean that you'll automatically win, you still need to make the right choices along the way.
With this setting turned off, it'll just be a normal random deal, and in some games that means it might not be winnable no matter which choices you make. It all depends on whether you want the original (and sometimes frustrating!) solitaire experience - just like with a real deck!
A couple of notes:
Clock doesn't have this setting - it's a game where there aren't really any choices to make, so it's what might be termed a mechanical game. It is winnable... sometimes! (We promise!)
Complex games - they will do their best, but as the game literally plays through deals using your chosen rule set, playing all of the ways to go through the game until it finds it can win (or not!), it can take quite a long time before it finds a guaranteed solvable seed. In general, if you choose tougher and more restrictive rules, it'll take the game a longer time to find you that guaranteed solvable deal.
When you start the game with this setting turned on, it'll show you progress as it looks for deals that are winnable. At any time you can 'skip' and go straight into a game anyway, and take your chance!