J.I. Packer: “The work of the Spirit in imparting this spiritual understanding is called illumination or enlightening. It is not a giving of new revelation, but a work within us that enables us to grasp and to love the revelation that is there before us in the biblical text as heard and read, and as explained by teachers and writers. Sin in our mental and moral system clouds our minds and wills so that we miss and resist the force of Scripture. God seems to us remote to the point of unreality, and in the face of God’s truth we are dull and apathetic. The Spirit, however, opens and unveils our minds and attunes our hearts so that we understand.” (Concise Theology p. 155)
Another definition: “The ongoing work of the Holy Spirit in the Christian person and community in assisting believers to interpret, understand and obey the Scriptures.” (IVP Pocket Dictionary of Theological Terms)
It is important to note, as Packer touched on in his definition, that illumination is not about receiving new revelation. The canon is closed (Revelation 22:18-19) and Scripture is complete and sufficient (2 Timothy 3:16-17) to meet our every need. Far too many people have been led astray by looking for new revelation from God instead of looking to the Holy Spirit to shed light and illumine what has already been given in the Bible.
So we would say that God not only speaks through His Son (Hebrews 1:1), but God also speaks through His Word. Through the agency of the Holy Spirit, God allows us to see the truth of Scripture that has been placed before us. A lot of people today emphasize the work of the Holy Spirit over and above the Word of God. In all reality, they work together. As we open the text of Scripture and search out its riches, the Spirit brings life and understanding to our hearts.
For further study read: 1 Corinthians 2:14-16, Colossians 1:9-10, 1 John 5:20, 2 Corinthians 4:6, and Ephesians 1:18.