Each invoice can have one of the following statuses:
When you go to all invoices view, in the right sidebar you will notice some statistics, also regarding invoice statuses.
When you create a new invoice, you can choose a status, but by default, it is a Draft. After you send your invoice, it automatically changes its status to Sent. After you click Add Payment and add a full amount from the invoice, it changes into Paid. This is the basic flow of the invoice. You also can change the status by editing the invoice.