An invoice is a record of the receipt of payment for an order. Creating an invoice for an order converts the draft sales order into a permanent record of the order, which cannot be canceled.
Only sales orders that are 'finalized' can be invoiced.

On the right-hand side, there are two options for you to create a '+ manual' or 'invoice all'.
Manually create an invoice

Using the '+ Manual' option, you can select parts of the sales order to invoice e.g. only specific line items or lesser quantity, etc.
Invoice all at one go

Choosing the '▶▶ Invoice All' option automatically creates an invoice with all the uninvoiced line items in your selected sales order.
If you wish to edit the invoice you created, simply click into the invoice under the Invoices section. Click on 'Edit' to make any changes to the invoice.
Looking to edit an invoice that has been paid? Learn how in this article.