There are at least two ways to answer this question.
On one level, we ask what these things literally mean. Literally, getting an assignment done on time means having finished a job that you have been given within the allotted time. A high level of quality is one in which the job has not simply been done at the minimum acceptable level. Instead, it has been done with a great deal of attention to detail.
On another level, we can ask what doing these things means to a worker and a firm. Put simply, they mean the difference between success and failure. A worker or company that does not do high quality work and complete it on time will not prosper. Other employees will be promoted over them and they may be fired. Other businesses will "steal" their customers with better and more timely service. In this way, completing assignments on time and at a high level of quality is the most important thing for both a business and an employee.