It is a style of preaching that preaches from a text of Scripture. The text determines the message and what is said. As one preaches this way, he seeks to answer as he preaches four questions: 1) What does the passage say (in its context), 2) What did the passage mean to the original hearers, 3) What then does the passage mean to me, and 4) What then should I do about what I have just heard?
The following link is helpful to further explain expository preaching: https://www.thegospelcoalition.org/blogs/erik-raymond/what-is-expository-preaching/
Pulpit Supply for Your Church 