Explanation/Reference:
The Add-SPSolution cmdlet adds a SharePoint solution package to the farm. This cmdlet does not deploy the uploaded SharePoint solution. Use the Install-SPSolution cmdlet to deploy the SharePoint solution in the farm.
Example:
Install-SPSolution -Identity contoso_solution.wsp -GACDeployment -CompatibilityLevel {14,15} This example deploys the installed SharePoint solution installs a previously added solution so it can be used correctly in both 14 and 15 mode site collections.