Your character's capabilities – such as athletics, swordplay and lore – are defined by 'skill' traits. The higher the rank, the better the character performs when using that skill.
When your character tries to perform an action with a significant chance of failing, you must roll a 'skill check'. To do it you roll a number of dice equal to the character's skill rank. Your character succeeds if you get at least one 'success'.
And what is a success? It's when one of the dice lands a number greater than its average. For example, if you get four or more in a six-sided die, or six or more in a ten sided-die. If you flip a coin you succeed when you get heads. So, as you can see, you can use any type of dice to play the game.
If you happen to get two successes in one roll, the outcome should be interpreted as being particularly good, and even more so if you get three – which should lead to a rare and serendipitous occurrence. No successes, of course, means your character failed.
Average skill and below
According to the neutrality principle (we'll get to it in a moment), a zero rank in a skill means you are average at it – and you'll probably be average in about half the skills available. If you have zero rank, you will automatically succeed in any trivial action which requires no checks, but will automatically fail any action difficult enough to warrant a check.
Of course, your character can also be below average. We refer to it as having -1, or 'x', rank. Such a character would blunder tasks that should be relatively easy to normal people. An x rank lore would mean your character can't read, while an x rank athletics would mean he can't ride a horse or swim.
Very often, though, checks will have 'modifiers'. Particularly difficult circumstances may lower the skill of a character trying to engage in an action, and a particular advantage may raise it. For example, climbing a barbed wall could give the character a -1 penalty to his athletics check, but having climbing tools could give him a +1 bonus.
If you ever have to face a skill check while having zero rank in it, getting modifiers is the key. They are commonly acquired by interacting with the scene and thinking out of the box. For example, a character with zero or x rank in athletics but with a solid craft skill might improvise climbing tools out of rubble, getting a bonus that allows him to proceed. By carefully studying an opponent army's strategies, you might get a +1 modifier in reflex checks against their arrows, for example.
Other than that, the most common way to get modifiers are through tools and weapons, and from merits and abilities, which will be dealt with later.
Most of the checks you'll roll aren't against a wall, but against another character that opposes you. If you try to hit someone, he'll try to dodge, if you try to manipulate him, he'll try to see through your lies and if you try hide, he'll seek to find you. That's what we call 'resisted' skill checks.
There are no secrets to it, really. If someone is trying to resist you, he will roll the appropriate skill check and each success of his cancels one success of yours. So you best him if you can score one more success than he could.
When two characters with zero rank are taking a resisted check, it is just a draw. The storyteller will decide what happens, based on the idea that no party got an advantage over the other (if you were trying to hide, for example, you weren't caught, but the sentinel is still suspicious). The same goes for two characters with x rank, although in this case there would probably be a comical ring to it.
When a character with an x rank is facing a character with rank zero or more, he offers his opponent a +1 modifier, so that even a zero rank has the chance of winning against him.
There is one more aspect to consider regarding checks. Sometimes the action you want to perform takes a lot of effort for a considerable amount of time, such as when you are performing in a festival, searching for someone in a city or holding the ceiling long enough for the others to escape. Such tasks might require multiple checks, and you'll have to be successful in most (or all) of them.