This is the 56th most frequent Serbian word.
под
"Под" in Serbian means "floor" in English.
In this sentence, 'под' is part of the prepositional phrase 'испод стола,' where it specifies the position of the book in relation to the table.
Књига је испод стола.
The book is under the table.
'Под' in this sentence is used in the prepositional phrase 'под мостом' to describe the movement beneath the bridge.
Прошао је под мостом.
He passed under the bridge.
Here, 'под' is a noun meaning 'floor,' referring to the part of the room walked on.