In short
The Gift of Prophecy
Prophecy is one of the gifts of the Holy Spirit. Adventists believe this gift was shown in the ministry of Ellen G. White. Her writings speak with prophetic authority and give the church comfort, guidance and correction, and show that the Bible is the standard by which all teaching is tested.
Read it in the Bible
Joel 2:28, 29; Acts 2:14-21; Hebrews 1:1-3; Revelation 12:17; 19:10
This is a short summary. The church's full statements are at adventist.org, and our pastors are glad to study any belief with you.