What do you mean by GSTR 1, GSTR 2, GSTR 3 & GSTR 3B?

GSTR-1: return for outward supplies i.e. sales made, services supllied.
GSTR-2: return for inward supplies ie.purchases made, services received.
GSTR-3: return containing details from other monthly returns filed by the taxpayer (GSTR-1, GSTR-2, GSTR-6, GSTR-7)
GSTR 3B: return for payment of Net GST liability (after adjustment of Input tax credit)