Discrimination is the ability to tell the difference between object/persons and responding to them differently.
In this statement, "discrimination" can be interpreted in more than one way. You want the test to discriminate between potentially capable employees and those that will be inept.
If it is valid, it will "measure what it was intended to measure." To be valid, it has to be relatively reliable. On the other hand, just because it is reliable, does not necessarily mean it is valid. The tests need to be picked to measure the desired qualities for adequate/good job performance.
On the other hand, you do not want it to discriminate on the basis of gender, age, race, religion, ethnicity, sexual orientation or other factors that will not apply to efficiency in the employment. Differences in the background experiences (e.g., education, informal training) or group characteristics may interfere with test scores, but not job performance. For example, older people typically do not perform as well on speed tests as younger people. However, if speed is not a component of job performance, a speed test will unfairly discriminate against older people.
I hope this helps. Thanks for asking.