Yea sounds like "yay" and is another word for "yes." It's often paired with "nay" when taking a vote in formal government or law and sometimes when asking friends if they want to get together, "Yea or nay?"
When an official asks for a vote on a bill or judgment, the question may be "How say you?" in the old style of English with an equally old-school response of "Yea" meaning a vote in favor. Using yea in casual conversation is less common but it's useful for showing that a person's "yes" counts as a vote because it matters in making a decision, as when you ask each other "Are you in or out — yea or nay?" when making plans.