It's a tool used for abuse that is meant to cause the victim to start doubting reality and thus, themselves, through subversive techniques. Generally, it's used to change the definitions of terms and is typically used by the alt-right.
Gaslighting is a form of psychological manipulation that seeks to sow seeds of doubt in a targeted individual or in members of a targeted group, making them question their own memory, perception, and sanity. Using persistent denial, misdirection, contradiction, and lying, it attempts to destabilize the victim and delegitimize the victim's belief.[1][2]