Thanks for asking!
It creates a timesheet based on your git history, and then creates an invoice from that. It guesses the number of hours you've worked an any given day, and also shows you your work for that day and allows you to correct the estimate.
It used to take me an hour to create an invoice - now it takes about a minute.
It's not a huge savings - but for those like me who dread creating invoices it makes me happy to get it done quicker.
Of course the tool is worthless unless you are a programmer checking all of your work into git. I'm trying to find out if this thing could be useful to people other than me.