Only Fighters and other warrior classes such as Paladins and Rangers can have 18/xx Strength as opposed to a regular 18 Strength, which is what other classes such as Thieves and Clerics are limited to. I think of the extra strength as a bonus for exercising and focusing all of their training on physical workouts. 18/100 Strength (which is usually abbreviated as 18/00) is considered "maximum human strength" or "ogre strength". In other words, a character like Minsc who has 18/93 Strength is almost as strong as a 9-foot tall ogre. That's pretty strong. If you ever watched the opening scene in the original Baldur's Gate game, there's an armored figure who holds another man over the edge of a tall building with one hand--he has 18/100 Strength. (In the movie
Commando, Arnold Schwarzeneggar's character lifted a skinny guy by his ankle with one hand and held him over the edge of a cliff. Later, when he was asked what happened to the little guy, Arnold replied, "I let him go.") 19 Strength makes a character as strong as a 10-foot tall hill giant. As you can see, here's a really big difference between a Cleric's 18 Strength and a Fighter's 18/100 Strength.
Half-orcs can start with 19 Strength at character creation, which makes half-orcs a popular race to play. If you have a party of 1st level characters with, say, a human Thief, a human Monk, an elven Mage, a dwarven Paladin, and a half-orc Barbarian with 19 Strength, it's pretty easy to see who can do the most damage in combat just because of sheer strength. There's a humorous web comic called
Dungeon Crawl loosely based on BG1 that does a good job of putting that in perspective.
But like kmonster said, the manual will tell you just about everything you need to know.