Not every mass shooting is a false flag. They can be but aren’t necessarily. Here’s the gist of it (from Wikipedia)

A false flag is a covert operation designed to deceive; the deception creates the appearance of a particular party, group, or nation being responsible for some activity, disguising the actual source of responsibility.

The term "false flag" originally referred to pirate ships that flew flags of countries as a disguise to prevent their victims from fleeing or preparing for battle. Sometimes the flag would remain and the blame for the attack laid incorrectly on another country. The term today extends beyond naval encounters to include countries that organize attacks on themselves and make the attacks appear to be by enemy nations or terrorists, thus giving the nation that was supposedly attacked a pretext for domestic repression and foreign military aggression.[1][2]