What does it mean for God to be "sovereign"?
In "Doctrinal Discussion" William Sillings writes:
Sovereignty can be defined as the essential quality of God by which He exercises His supremacy because of who He is and in harmony with whom He has revealed Himself to be. His sovereignty does not allow Him to act outside of His other essential characteristics. That is, being sovereign, He is also holy, just, loving, eternal, etc. His sovereignty does not detract from or diminish His other attributes.
For practical implications of the sovereignty of God, see the "Doctrinal Discussion" in Adult Teacher's Insights!
Source: Studies in the Psalms: Adult Teacher's Insights, page 8.