Dr. Donald McGavran's definition of a homogeneous unit is "a section of society in which all members have some characteristic in common."
In plainer terms, a homogeneous unit is a group of people that have ethnic, linguistic, social, educational, or vocational similarities.
In terms of a church congregation, ethnic, social, and educational commonalities are the most important.
People feel more comfortable when they feel they are part of a group. I suspect that people are comfortable in a church where they don't have to cross racial, linguistic or class barriers. In my county in the United States, there are white United Methodist Churches and there are black United Methodist Churches. I attend a white church. I've visited a couple of the black churches as a lay leader, but I'm more comfortable worshiping in a white church.
The invention of the automobile allowed people to ignore geography (parish) and worship in a church they feel more comfortable in (social, educational).
The principle of homogeneous units describes what almost anyone can observe. People like to gather as tribes, and feel part of the group (in). Unfortunately, there are also people outside the tribe (out). Part of Christ's message was to accept all people into one tribe. That part of the message is lost on some Christians.
Some of the information in this post came from The Pasadena Consultation - Homogeneous Unit Principle.