Gratification is the pleasurable emotional reaction of happiness in response to a fulfillment of a desire or goal. So Instant Gratification is where you lost something on Facebook or YouTube and immediately you get some fulfillment in the form of likes. This culture where people need instant gratification has pushed kids to do stupid things like the recent tidepod challenge.
Some even suggested that mass shooting in the country are also linked to this, cos these sick people also seek instant gratification and thus instead of going to friends and share their anger, they do these heinous things.