SUMMARY: Well--not all about--just some useful stuff in general and Tika's in particular.
Tika has congestive heart disease. Depending on which veterinary web site you visit, either cancer or congestive heart disease is the leading cause of death in dogs.
So, what's going on in her heart?
Here are the parts of the heart. A dog's heart is essentially the same.
(image from this site.)
Found this video that shows how the heart works, and a dog's works the same way.
The vet kindly drew a sketch on a paper towel of what's going on with Tika's heart. High-tech! This drawing is reversed from above--Left Atrium (LA) and Ventricle (LV) are on the left here.
Tika's mitral valve--between LA and LV--has become enlarged and hardened. So (a) it doesn't close all the way and (b) it's prolapsed--it opens in the wrong direction as well (picture a door that should open inward now opening outward). So, when the heart pumps, instead of the proper fluid staying in the proper direction as shown in the video, some of the fluid squishes backwards, increasing the pressure in the Left Atrium. (That's about as technical as I can get.)
The additional pressure has caused her Left Atrium to become greatly enlarged--the dotted line--which means that the muscle wall is stretched badly and no longer pumps as well; also, it's encroaching on the Right Atrium, making that part work harder to do its job and to get enough blood flowing.
What happens eventually is that the valve or the left atrium stop working entirely, for various reasons. In that case, it's all over, and it could be very sudden.
Some places have done valve-replacement surgery on dogs, but not that many and (a) it's really, really expensive, (b) very involved surgery with lots of risks, which I don't really want to put Tika through, (c) Tika's left aorta is in bad enough condition that it's not really worth it to try fixing just the valve, and (c) she's nearly 12, so even if everything else were to go right, I don't know that I'd get more than a year or 2 anyway.
Example discussion: http://www.vetinfo.com/vets/answers/can-dogs-get-heart-valve-replacement
And now you know.