The Product Grid is where you can enter Products and details such as Buy and Sell Price, Mark-up and Discounts, on Quotes, Orders and Invoices:
This is not turned on by default on Opportunities, however you do have the option to switch this on. This can be done by going to Settings > Additional Settings > Opportunity Settings.
Enable the following setting (highlighted in green) and click Save:
This will turn on the Product Grid. With this enabled any Product details that are captured at Opportunity level will automatically be carried across to any subsequent sales entities (Quote, Order, Invoice) that you raise directly from this Opportunity.
You can also use the Products sub-total to automatically populate the Opportunity Amount, which is the next option in the Opportunity Settings.
Once this is enabled you are not able to manually enter the Opportunity amount, it will always use the Products sub-total: