It is a join condition containing something other than an equal operator. The link between emp and job_grade table has an exampleof a nonequijoin. The relationship between two table is that the salary column in the emp table must be between the value of lowest_sal and highest_sal column of the job_grade table. The relationship is established using an operator other than equal(=).
The above example is based on non-equijion, which will display the employee salary grade based on there salary. The salary must be between any pair of the low and high salary range.