Strength determines one's Hit Points and maximum Weight Points. One's Strength is equal to their Hit Points. One's maximum Weight Points is 18 times their Strength. Strength is determined by Class and Class Level.
Fighters and Rangers have the most Strength. Druids have the next greatest, followed by Rogues and Wizards. Clerics have the least Strength out of all the classes.
Fighters, Rangers, and Rogues can use the Anatomy skill to determine the range of one's Strength. Doing this only gives a rough estimate of someone's Strength; the exact values can be found on the Anatomy page.
Strength is unrelated to Food, although food has a role in gaining Successes.
The Strength spell and the Strength Potion temporarily increase one's Strength stat. People sometimes rely on using these so that they can temporarily carry more items.