Reliability refers to the consistency of a measurement. A reliable test will produce similar results under consistent conditions. Think of a bathroom scale: if you weigh yourself multiple times in a row, and it consistently gives you the same weight (even if that weight is wrong), the scale is reliable.
Validity, on the other hand, refers to how accurately a test measures what it's supposed to measure. Using the same scale example, if the scale consistently gives you a weight that's 10 pounds heavier than your actual weight, it's reliable (consistent) but not valid (inaccurate).
Therefore, a test can be highly reliable in producing consistent scores, but those scores might not accurately reflect the construct the test intends to measure, making it invalid.