What does karat measure?
Karat expresses how much of a gold item is actually gold, in parts out of 24. Pure gold is 24 karat; 18-karat jewelry is 18/24, or 75% gold, with the balance alloyed for strength and color.
Don't confuse karat (purity) with carat (gemstone weight), same sound, different worlds.
Why do alloys exist at all?
Pure gold is soft, it scratches, dents, and wears. Alloying with copper, silver, or other metals makes a product harder-wearing. The American Gold Eagle is 22 karat (.9167 fine) for exactly this reason, yet still contains a full troy ounce of gold in a slightly heavier coin.
Lower karat doesn't mean less gold value when the weight accounts for it, what matters is total gold content, which is why fineness and weight together tell the whole story.
How does karat show up in practice?
You'll meet karat in jewelry, older coins, and the Gold Eagle; you'll meet fineness on modern bullion bars and coins. Convert between them by dividing by 24: 22 karat ÷ 24 = .9167 fine.