Silent aspiration occurs in people that have swallowing problems, which is known as dysphagia. Aspiration is when food, liquids, or stomach contents are swallowed poorly, and go into the lungs
by mistake. Usually when this happens, normal healthy people will turn red and start to cough, as an attempt to get it out of the lungs (think about yourself, when "something goes down the wrong pipe"). However, as people get weak and swallowing worsens, this aspiration can happen silently, which means there will be no signs showing that this has occurred. In other words, you will not see any coughing, choking, or turning red.
Aspiration is dangerous because the food/liquids/stomach contents enter into the lungs, which is very irritating. It can develop into an aspiration pneumonia, which is very hard to treat, even with multiple antibiotics.