A solution can contain multiple projects that you can open, close, and save together. Each project in a solution can contain multiple files or items. The types of items contained within a project depend on the script language used to create them.
The logical relationship between a solution and its components does not necessarily mirror the way the solution and its components are stored on disk. If you create an application consisting of multiple projects, you might consider creating a solution directory in storage to contain your local projects, solution files (.sqlsln and .sqlsuo), and any shared solution items.