I thought it was when a software code is released to public, so that users can build it up??? Am I on the right track? If so, what does this mean for blender?
You are right.
It means users and other people in the Open Source community will be able to add features to Blender.
CAVZ24, you might want to have a look at the Open Source Definition website of the Open Source Initiative, and the Free Software Definition website of the Free Software Foundation. Also check out the Blender Foundation forum for some interesting discussions regarding Blender and Open Source.
And always remember: software can be both Open Source and Free Software at the same time, but the two movements have (big!) philosophical differences, as explained in the Free Software Definition.