The prepositional case in Russian is used with a preposition for location, when you are talking about someone, or for the names of months when talking about time. This video primarily addressed answering the questions Где ты? and Где вы?.
The prepositional case of a noun is used to designate the place when answering the question "where" with the prepositions в, во and на.
Here is a table that shows you how to form the Russian prepositional case endings, as well as discusses other reasons to use the prepositional case.