How it works depends on what kind of study it is. If they're investigating a new drug or combination of drugs, she might be offered the opportunity to be treated either with the current standard medication for her condition, or a new drug, or a combination of the current standard medication plus something extra -- neither she nor her doctor would know which. She would need to have followup visits to determine how she's doing. Eventually, when enough patients are involved and there is evidence one way or another, she and the doctor would find out which regimen she had been given, and all the data from all the patients would be analyzed to determine which patients in which group benefit the most, which have which side effects, etc..
These trials are designed very carefully and the health of the participants monitored closely -- studies are occasionally stopped because of unexpected negative OR positive effects. For example, a few years back when it was discovered that the cardiac patients who took aspirin had many, many fewer heart attacks and strokes than those who didn't, the study was halted and the results published because it would have been considered unethical to withhold that information any longer than necessary -- the results were just that overwhelming.
The doctor will have documentation from the study trial that is designed to help patients understand exactly what the goal of the study is, and whatever else is expected. Sometimes treatment/medication is free in exchange for participating.
Usually there are very strict criteria for patients to be enrolled, so it sounds like the doctor feels that your Mom is exactly the kind of person who might benefit from whatever the clinical trial is studying. She will still retain the full right to stop participating at any time.
. He is trying to keep busy in his retirement years. 
