Reductionism is the act of oversimplifying an issue, breaking it down into small parts that don't reflect how complex it actually is. Political scientists might accuse journalists of reductionism when they briefly sum up a complicated topic.
You might come across the term reductionism in a philosophy class — in this context, any scientific theory, object, or meaning can be reduced to its individual parts. If you understand these smaller components, you will understand the larger concept. A more derogatory way to use the word is to accuse someone of trying to make something too simple through reductionism. Someone who tends to do this is known as a reductionist.