what is reverse charge mechanism?

under section 9(3) the gov. notified some supplies for which the tax will be payable by the receiver and not by the supplier .
this is rcm
under section 9(4) it is provided that if u procured goods or services from unreg. dealers and if u r reg. then u have to pay tax under rcm.
exept salary electricity staff welfare almost all the exp are covered if the supplier is unreg. and recepient is reg.

In case RCM is applicable to you, you have to show in GSTR 2