We come to find out the husband is actually just using the marriage in order to find something valuable that belongs to her. She believes she's going crazy because what happened in those days was if you turned on a light in one room, it would dim all the other lights throughout the house (because there was only a certain amount of gas that could light the whole house).
As a result, you would know someone else was in the house or in another room, turning on the light, if you saw the lights flicker. And so she noticed that the lights would flicker every night after her husband had gone away somewhere, (supposedly to see friends or whatnot). He was actually rifling through the attic but he convinces her that he is imagining what she is clearly seeing (the gaslight flicker). When in reality, she knows better, but she believes him anyway.
That’s what it means to gaslight somebody — to convince them to deny what they know to be true. Gaslighting from an abuser makes a victim doubt their own instincts and sense of reality.
* The 1944 film is the American version of the original 1940 British film, which in turn is based on the theatrical play. We reference and feature the 1944 film since it is more well-known among American audiences.