I see listings for analog FPV cameras like this one which have ratings in units of TVL. (e.g. 1000 TVL or 1200 TVL) What is TVL?
TVL (TeleVision Lines) is a measurement of the "resolution" of an analog camera. Digital video systems use pixel resolution to specify how many pixels wide and tall the image is (e.g. 1080p = 1920 pixels wide by 1080 pixels tall), but analog video systems don't technically have pixels so there has to be some other way to represent their resolution capabilities.
The horizontal resolution power of analog video (TVL) measures how many distinct vertical bars can be depicted by the device (either a camera or display screen) across a horizontal length equal to the height of the picture. The example diagram above shows analog video with 6 TVL.
Cameras with higher TVL ratings will be able to capture smaller details than other cameras, although this is also dependent on the capabilities of the other equipment in between the camera and your FPV goggles or screen.