The On Delete and On Update dropdown list define the type of the actions to be taken.

Restrict
Produce an error indicating that the deletion or update would create a foreign key constraint violation. This is the same as NO ACTION except that the check is not deferrable.

No Action
Produce an error indicating that the deletion or update would create a foreign key constraint violation. If the constraint is deferred, this error will be produced at constraint check time if there still exist any referencing rows. This is the default action.

Cascade
Delete any rows referencing the deleted row, or update the value of the referencing column to the new value of the referenced column, respectively.

Set Null
Set the referencing column(s) to null.

Set Default
Set the referencing column(s) to their default values.