What are the nouns in the following sentence?: The three red bicycles are in my garage.
One way to think of nouns is to see them as "persons, places, and things" (it's not exactly right but it'll work most of the time).
In the sentence
The three red bicycles are in my garage.
which words are people, places, or things?
Bicycles. The words leading up to "bicycles" tell us more about them (The tells us we're working with these specific bicycles, three tells us the number, red tells us the colour).
Garage. This is the place where the bicycles are. The words leading up to "garage" tell us more about their location (are is a form of the verb "to be", in tells us that the bicycles are within something versus "near" or "on" or "under" or any other preposition, my tells us whose garage we're talking about).