Referencing (or citing) is a formal way of acknowledging the sources of information, ideas, theories, statistics, images, musical phrases, diagrams, etc., you have used in your essay or assignment.
A correct reference (or citation) gives all the details used to identify where you found the information you used in your assignment, so that the source can be verified.
For example, for a book the citation would include the author(s) name, year of publication, book title, edition (if relevant), publisher and place of publication.
By referencing your sources correctly you are practising Academic Integrity.
There are many different referencing styles which can be used to reference (or cite). Ask your teacher which style you should use.