'Sandbox' is a node js module that emulates the ethereum network. Once you have a contract written, run it and, the Sandbox panel appears on the right side of the screen.
 This pin enables you to lock the Sandbox ID# so you don't have to edit your app.js each time you run your contract(s).
 SandboxID#. Each time you start the sandbox, a new SandboxID is generated. Before you can view your app in your browser, you must verify that the app.js file has the correct sandboxID listed. If you don't want the SandboxID to change, pin it (see note 1)
 A contract account with an ether balance and data.
 Sandbox tab to open running Sandbox if hidden
 An address with ether balance. These addresses are configured in the project ethereum.json file. The purpose of these addresses is to populate the sandbox test blockchain with live addresses for testing your contract(s) annd app.
You can even 'Send contracts to the net' by using the button on the top menu tool bar above the Sandbox.