An elder is any person who is older than you, which you may know from your sister who is only two years older than you saying, "Listen to your elders!" A church elder is someone who helps run the church, not necessarily its eldest member.
Elder can be used as an adjective: Abel was the elder brother of Cain. Elder can also be a noun: showing respect for one’s elders is a value instilled from a young age by our parents. Elder is also a kind of tree. In Act Five of “Love’s Labour Lost” Shakespeare plays with the meanings of elder: Holofernes says that Berowne should speak first because he is the elder (older), which inspires Berowne to mention the folklore that Judas was hanged on an elder tree.