What is the meaning of reverse charge under gst

Any person who paid tax to government instead of pay tax to seller.

Ex:-  registered person and unregistered person

1. Registered person purchase goods or services from unregistered of Rs.  100000/-

Here registered person will pay amount to unregistered person but not pay tax to  unregistered person 

So, tax deposited to govt. By registered person

Since, here reverse charge mechanism apply to registered person he will write on book reverse charge