At a very basic level, the film loses sensitivity after it's maximum exposure has been reached, manifesting as poor contrast, colour shift/cast and other anomalies. You can see reciprocity failure in action if you use, for example, Provia 100F (slide film) for star trails. Set the camera up, trigger the shutter and lock it open for say 6 hours. This exposure time is way, way beyond the film's design intent for correct exposure over a given range. What happens over that long period of time is that the colour shifts to a strong magenta (with Velvia, it is a strong green) and gains contrast. Provia 100F, along with Velvia (used correctly) is a beautiful film with no casts whatsoever. It's what happens when you make very long exposures how the film's design breaks down. It's often exploited by traditional arts students in the street / documentary photography oeuvre.

You can hedge against reciprocity failure by providing additional exposure, but at some stage RF will still creep in. There is no camera that corrects for reciprocity because it is a characteristic of film, not one that a camera has control over (but to some degree, the photographer does, for the same statement above, by providing additional exposure and corrections). RF is a journey of discovery and is well worth experimenting with when you have selected a film you like and which you will use quite often, getting to know how it behaves beyond it's limits is one of the more interesting investigations you can do.