A Sales Stage indicates the level at which an opportunity is in the sales cycle.
Generally high values deals go through a number of stages before the sale is concluded. You will have to nurture high value opportunities over a period of time before you can successfully close a sale.
You can create the sales stages and in each opportunity you can select the stage it is in and update it as you nurture the opportunity. This helps you getting an overview of how many opportunities you have in each stage.
To access Sales Stage, go to:
Home > CRM > Settings > Sales Stage