The word "yeah" is an informal way of saying "yes." It's used in casual conversation to express agreement, affirmation, or acknowledgment. People often use "yeah" instead of "yes" when they want to sound more relaxed or friendly.
The word "yeah" is believed to be a variant of "yea," which has its origins in Old English "gēa." The informal version became popular in spoken English by the 20th century, often used in colloquial contexts.