Prepositions are words that show the relationship between a noun or pronoun and another word in the sentence. They usually come before a noun or pronoun.
For example:
* "The book is on the table." Here, "on" is the preposition, showing the relationship between the book and the table.
The word "supply" itself is a noun or verb, not a preposition. It can be used in a sentence with a preposition, but it doesn't contain one within itself.