Physical stats are straight-forward and concrete enough that people should get them easily so I won't even mention them. I'll focus on the mental stats since they are so much more abstract and, quite frankly, very difficult for people to wrap their heads around (especially Charisma).
Intelligence: How easy it is to figure out new information and how easily you can recall previously learned knowledge. This is probably the easiest of the mental stats. High intelligence means you are knowledgeable, learned, and have enough memory to recall this info. Keep in mind, you aren't necessarily good at putting this info into good practice or with "intuitive" knowledge (gut instinct). A classic example is the absent-minded professor; high Int but low Wis makes him a literal storehouse of knowledge with little in the way of common sense. When applied to a spellcaster, they are "hacking" the magic pathways of the world, forcing them into certain desirable configurations based on knowledge and understanding of cause/effect relationships.
Wisdom: How easy it is to apply what you know to "real-life" situations and how well you discipline and control your mind. High wisdom means you have a strong, disciplined mind and are good at "practical applications" of what you know. You may not be a storehouse of eclectic knowledge, but you can apply everyday things in everyday situations. This also covers knowing what things "should" be like so that you can notice when they are not as they should be (perception, sense motive, etc). For Spellcasters, it is about seeing the natural pathways as they are and invoking natural responses with minimal tampering, usually conveying prayers and wishes to spiritual and/or divine beings.
Charisma: This is the hardest for people to wrap their minds around for two reasons. 1) It covers many things that, at first glance, seem disparate. 2) The unfortunate wordage used in the book, linking "appearance" (a physical characteristic) to this ability score. In studying the system, I've found that the best word to boil Charisma down to is confidence. Charisma is a matter of confidence in yourself, and confidence that other people will behave predictably. High Charisma means you have a great force of Ego, a great confidence in your actions, your words, your demeanor, and a confidence in others to react to you accordingly. Low Charisma means a lack of confidence, a person plagued to a greater or lesser extend, by pessimism. Naturally, someone is going to bring up, "How does appearance work into it"? Well, first off, even linking a subjective term like "appearance" to a number, you must keep in mind that it isn't a matter of a bigger number means a more likable appearance while a lower number means a less likable appearance. The number isn't a determination of the subjective quality of appearance (whether you are pretty or ugly) but rather a quantitative measure; whatever qualitative description applies to the character, Charisma measures how strong that quality is. So if it's an ugly character, high Charisma means very ugly and low Charisma means still ugly, but just in a "eww, gross" sort of way rather than a mind-shatteringly grotesque sort of way. To illustrate, consider a pair of beautiful blonde twin babes, physically identical in all ways, but with differing Charisma scores. Even though they each look the same, the one with the higher Charisma score will be perceived as more beautiful. How does this work? Because confidence. She is more confident that she is, indeed, a beauty and people very well should be smitten with her. She has a confident way that she carries herself, speaks, and acts and other people subconsciously respond to these subtle cues of body language more strongly because they convey a greater sense of confidence, as compared to the less confident twin. Another example is telling jokes. Some rude, crass guy tells an inappropriate joke and, because of his personal confidence (high Cha), uproarious laughter is the response. A different, equally rude, crass guy could tell the same joke to the same room (pretend they haven't already heard the joke), but they don't find it as funny. He didn't stutter or flub it, but his delivery left a certain subconscious component wanting. So high Confidence isn't necessarily talkative nor polite and low confidence isn't necessarily rude nor crass; both measures of the score can cover any and all qualities of expression. The Cha score only determines how powerful and effective those qualities of expression are. Another thing to consider is that Disguise, the ability to draw notice away from your actual physical features, is a Cha skill. You need Charisma to make yourself less attractive than you really are because there will be that much of a difference in subconscious response.
