Spoon feeding is used both idiomatically and literally. It is literally feeding someone (typically a baby) with a spoon by placing food on the spoon and then putting the spoon in the recipient's mouth until they remove the food using their tongue or lips. The only action required by the person being spoon-fed is to close the lips after the spoon has been inserted into the mouth and swallow the food.
In the more idiomatic or metaphorical use, it refers to someone doing something for someone else to such a extent that the person being "spoon-fed" has to do only the absolute minimum. Since most people have experience with how babies are fed, even if someone is not immediately familiar with the metaphor, it is pretty self-explanatory, unlike some idioms which make no sense unless you know the idiom. In the case of a class where students don't have to do any thinking, analysis, or practical tasks but just memorization of facts could be an example of "spoon feeding".