Workspaces can be used to organize related solutions, and provide a convenient way to open several solutions at once. Unlike projects in a solution, which can have references and build interdependencies on other project within the solution, solutions in a workspace are independent, and can be opened and built independently of the workspace and the other solutions in it.
To create a workspace, use the command File->New->Workspace, enter the desired name of the workspace, and select the location. The workspace will be saved as a mdw (MonoDevelop Workspace) file. To add existing items to workspace, right-click on it in the solution pad, and use Add->Existing Item. This allows you to add workspaces as well as solutions, since workspaces can be nested. There are also commands to Add->New Solution and Add->New Workspace, which create a new solution or workspace and add it to the workspace directly.