First among these is CTO: "correct to orange" or, as some would have it, "color temperature orange".
When shooting indoors with regular incandescent (tungsten) lights lighting the interior — well, they have been regular so far, but they have now been outlawed — and the strobe lighting the subject, you want to gel the strobe (flash) with a CTO gel. It will make the white light coming out of the strobe orange. Consequently, everything in your shot — both your subject and the interior lit with the overhead lighting — will have an orange tint. You then correct white balance in Lightroom, Photoshop or what have you. Done.
The alternative would be to cover all incandescent lights with a CTB ("correct to blue") gel. Then their orange light (we may not realize it's orange, but the camera's sensor does) would turn white and the interior in the shot you take with a flash will lose the ugly yellowish tint that you always struggle with in white balance correction ("If I correct the wall to white, how come the face turns blue?")
The problem with this latter approach is that you will have a considerably larger gel, perhaps several, a stepladder and a bunch of tape or clams to secure gels to every lightbulb in the room.
While with the CTO gel, you only need one, it's small, and it may even come with a handy holder that affixes it to the flash.
(I actually learned this over a year ago, but never finished the draft of this post.)
