Combat is a simple thing to play. Skills are even easier. Every now and then skill tests will be used to determine whether a creature succeeds in a certain task, apart from combat. These tests are carried by rolling the special skill die:
If a success symbol is rolled, the test has been superbly passed; if a failure symbols appears, the character has failed miserably, and the GM may even rule bad consequences as he seems fit. Otherwise, when a number is rolled, it will be added to the stat number of the appropriate skill. If it's equal to or higher than the difficulty number previously established by the GM the test is passed; when lower, it just fails. If you look at the die layout, numbers range from 1 to 4. Skills will be rated from 0 (worst) to 6 (best). And difficulty numbers will range from 1 (easiest) to 10 (most difficult).
The automatic success and automatic failure symbols are there to make the game even easier and faster (behind the scenes, I can tell you they are the legacy of Games Workshop's games "a natural 1 always fails and a natural 6 always success" rule). Yet the GM must remember than whenever the skill die is rolled there's always a chance of success and a chance of failure (16,67% each). But sometimes you don't want them to fail; for example, you ain't telling them to roll an agility test to see whether they stagger whenever they walk; in these cases don't make them roll dice. On the other hand, you want to prevent abuses; if an illiterate character (KNO 1) wants to make a knowledge test (difficulty 9) just hoping he rolls a success, tell him he can't even make the test. That would be just abusing the rules, and is thus forbidden; impossible things are out.
The just explained skill tests are the "simple skill test". There's also the "opposed skill test", when you don't roll against a fixed difficulty number given by the GM, but against the skill test of another creature. This is done with both creatures rolling the skill die and adding the result to the appropriate skill. Higher wins. Should one success or one failure appear, it obviously means success or failure. Should two successes appear, the GM will determine whether that means two simultaneous successes or a draft. An example of the former would be two wrestlers trying to grapple each other, both can success simultaneously, grappling the opponent yet being graped themselves too. An example of the latter would be someone chasing another, in which case they maintain the same distance. When two failures appear, both creatures have failed, and miserably; with the former examples, the wrestlers would have suffered painful luxations, and the chased and chaser would have stumbled and ended with their faces stamped on the floor.
Now let's have a closer look at what the skills are useful for.
Fortitude (FOR). This is a combination of strength and constitution. To ram down a closed door, lift a portcullis, swim across a strong river, or ward off the effects of venom this is the skill to use.
Agility (AGI). Good reflexes, body coordination and flexibility is what agility means. You will use this skill to chase someone or pass through a very narrow passage, for example.
Perception (PER). This skill stands for sensory awareness: the ability to see, hear or otherwise detect the presence of creatures or features of some worth to the adventure.
Dexterity (DEX). Not to be confused with agility, dexterity is eye-hand coordination and nimbleness. This is the skill used to make a "magical" trick, play a musical instrument, or create a counterfeit document, just to give some examples.
Intelligence (INT). The ability of rational reasoning, used to solve a problem. Deciphering a codified message and discerning the secret motivations of a person are a couple of things done with this skill.
Sociability (SOC). Social skills are what are needed to delve successfully in society, be it a classy palace or a downtrodden gutter. With a high score you'll be able to easily befriend unknown people, get information from them, or convince a seller to offer you a special low price. This skill will be key in the Necronomicon setting.
Knowledge (KNO). This skill is a bit special, in that it means the general knowledge a person may have; but also, for every point it has, it grants a specific area in which that person is an expert. It's the GM who will determine whether a piece of info may be known by passing a KNO test ("hey, I just remember hearing that..."), or whether some expertise will be enough to discern some piece of info ("let me tell you that this essay is just a plagiarism of the book...").