The AutoCAD DesignCenter is a repository of blocks, hatches, drawings, templates, and other drawing content that you can use to drag and drop content like blocks into the AutoCAD drawing editor. It’s easier and faster to do this from DesignCenter than it is to copy and paste items like blocks from other AutoCAD projects.
From DesignCenter, you can also do things like create Tool Palettes, redefine a block’s definition, and open the Block Editor. You can access Design Center from AutoCAD’s Ribbon in the Palettes panel of the View tab. For more in-depth instruction, visit this page.