I have searched for purge method but didn't found anything precise.
A purge is what happens when you pick out an object from the cache and discard it along with its variants. Usually a purge is invoked through HTTP with the method PURGE. An HTTP purge is similar to an HTTP GET request, except that the method is PURGE.
This is what I found.
That's correct, you can send the PURGE method to remove a resource from the cache. This is a example to illustrate this:
GET /article/1234 HTTP/1.1
GET /article/1234 HTTP/1.1
PURGE /article/1234 HTTP/1.1
GET /article/1234 HTTP/1.1