Hi,
I suppose you’ve enabled a mode when due dates are shown in relative format. There is a keyboard shortcut df for that (due format), and there is a corresponding option in Checkvist settings, too.
So basically, +1d means that the due date for this task is tomorrow 
Here is a discussion where we’ve decided to add such a mode: Is there a way to view due dates by days/weeks remaining?
Kind regards,
KIR