This event occurs when we need to grab more (or release some) memory for the operation currently being performed. Of course, the action of getting/releasing memory has always been inside Oracle - it is just than in more recent releases, we expose the event so you can see it.
To solve it - tune your SQL. That might sound vague...but its only as vague as the information you've given us :-)
If you really want to dig into the details, check out Frits's blog
https://fritshoogland.wordpress.com/2017/03/01/oracle-12-2-wait-event-pga-memory-operation/