Validity means actually measuring what you claim to be measuring for example don’t do an investigation to measure the temperature that water freezes up but instead of using water use milk.
Reliability means producing stable and consistent results. Repeatedly. For example, it wouldn’t be reliable if you conducted the test on Jupiter where water maybe doesn’t freeze at zero.
Bias is where the scientists or researchers performing the research influence the results (intentionally or not). For example, an interview about personal habits is conducted by an attractive interviewer and the respondent tells them lies as it may appear to be unattractive to them, so would mess up the final results.
What if you took a test and two weeks later took the same test? Will the results be the same?
Not necessarily, because you could be ill that day so it may be difficult to concentrate. Someone in the exam could be off-putting for example coughing non-stop or there could be rain or a loud aeroplane outside that might put you off and lose where you are. You may also have revised but forgotten by the next time. Your mood may be worse or you may have something on your mind the second time.