Mom (88 with dementia) had EEG today. What does a test like this show?

That IS a short time. You have a good doctor because he is ordering tests instead of brushing off your concerns. I'm glad that the CT Scan was unremarkable. There are many different reasons to ask for an EEG, since you know your mom best, I think that you would have a better shot than I would looking up information online about Electroencephalograms and what they are used for. You know what she was like before and what symptoms she is exhibiting now. Remember though, not to read too much into the information you come across, AND keep in mind that her CT Scan was clear. Write down and questions and concerns you have for the doctor as you are doing your research. They usually call back sooner when there is a problem, although if you have an appointment already scheduled, he may just wait for that.

I have had more than several EEGs through out my life, but that is because I have a seizure disorder, and Myoclonic Jerks/Twitches. You haven't mentioned anything like that, so I am guessing it's not her problem.

EEGs are used for things other than that.

Do you know why she had the EEG? What were they looking for?

She had declined significantly in a very short period of time (3 weeks). Going to the neurologist was my idea; he recommended a CT scan (which she had and was unremarkable) and today's EEG.