A more thorough explanation:

Definition: A cautioner is a person who provides a warning or gives caution. In legal terms, a cautioner is someone who puts up security to ensure the performance of an obligation.

Examples:

• A mother cautioning her child to be careful while crossing the street.
• A landlord requiring a tenant to have a cautioner to ensure rent payments are made on time.
• A person acting as a cautioner for a friend who is taking out a loan.

The first example illustrates the general definition of a cautioner as someone who provides a warning or caution. The second and third examples illustrate the legal definition of a cautioner as someone who puts up security to ensure the performance of an obligation.