An allusion is a figure of speech whereby the author refers to a subject matter such as a place, event, or literary work by way of a passing reference. It is up to the reader to make a connection to the subject being mentioned.
Example:
It’s no wonder everyone refers to Mary as another Mother Teresa in the making; she loves to help and care after people everywhere- from the streets to her own friends.
In the example the author uses the mention of Mother Teresa to indicate the sort of qualities that Mary has.