'Reliability expresses the reliability of the test. That is, whether we get similar results when using the test repeatedly. Reliability takes values between 0 and 1 (100%). The test has high reliability if it gives the same results when repeatedly measuring the same object.
Reliability expresses the technical quality of the test. Not its correctness. A test can be reliable - have high reliability, but may not measure the trait under investigation - so it can simultaneously have low validity. The relationship between reliability'' and validity is that between accuracy' and correctness. The reliability of the test is therefore a necessary prerequisite for its validity.