Sermons have been preached in Christian communities from the earliest days when the New Testament epistles and Gospels were still being written. To this day, preaching still plays a role in the life of the Church. normally, a sermon is preached by a bishop (if present), priest or deacon.
Some smaller congregations without weekly access to clergy may train lay worship leaders to preach sermons as well. Giving a member the authority to preach a sermon isn’t a matter taken lightly. This is why only designated people preach sermons.
Most of the time, the designated preacher* for the day preaches a sermon or homily (shorter sermon) about one or more of the lectionary readings. Some might try to preach a sermon based on all 3 lessons for the day, while others might focus most of the sermon on a single reading. Occasionally, you might hear a sermon about some aspect of our worship or a current social concern. The sermon might be preached from the pulpit, or from the center aisle of the church in the midst of the congregation.
*Preacher designates a certain job that clergy fulfill as part of their profession, however, in the Episcopal Church tradition, a clergy member is not generally referred to as “a preacher”, nor is the term used as a title.