A single invoice is just when you need to create one invoice for your client to send out.
An invoice batch is a group of multiple invoices all created at the same time. It speeds up your invoicing time and allows you to print all of them with a single click. This way, you don’t have to go through the same motions for every single invoice by the time you reach the end of your billing cycle. See the videos below for creating both Single Invoices and Batches: