According to Microsoft, Variables store values that a SQL Server Integration Services package and its containers, tasks, and event handlers can use at run time. The scripts in the Script task and the Script component can also use variables. The precedence constraints that sequence tasks and containers into a workflow can use variables when their constraint definitions include expressions.
Variable just like same as we use in our programming language but some different in variable declaration, we declare variable in ssis given bellow pic.
First click on the menu SSIS and then click on Variables it open a variables windows given bellow.
To create a variable, click on red pointed arrow. Here you can modify variable name, data type, value, expression as per your requirement Same as if you want remove/delete variable then click on blue pointed arrow.
After click on green pointed arrow it's open a "Select New Scope" window from here you may decide your variable scope. see in bellow pic.