What is and why use Sandbox Solution?
Microsoft had introduced SharePoint Sandbox Solutions for the first time in SharePoint 2010. Sandbox solutions enabled site collection users to upload their own custom solutions.
For those who do not have SharePoint Farm privileges, this solution proved to be very useful. All the user had to do was create/develop a Sandbox solution using Visual Studio, upload it to the solution gallery of the particular site collection and it was ready to use after activation.