When we use a measuring device in the lab to measure something, we need to make sure that our test is valid and reliable.
So what do they mean?
Validity:
Validity means our device measures what we want to measure in a fairly accurate way. How do we know that our device measures precisely? We might have some universal measurement and if our device shows a good correlation with that universal measurement, then we could say proudly that our measuring device is valid 
So, what you do is basically compare your measurement/test with another valid test.
Reliability:
Reliability is the repeatability of our test. That is, how consistent is our measurement every time.
Example 1: Supposing our device is a weight measurement device and each time it shows a different weight, say 100 lbs one time and after 10 minutes 150 lbs, can we rely on that measurement? The test-retest is not reliable.
Example 2: Supposing we are using a new-goniometer-kind of device to measure the joint angle for an individual. Person A is recording 50 deg, while person B is recording 70 degrees for the elbow flexion of the same individual. Can we rely on this device? Each person is getting a different output. The inter-rater reliability is bad.
The picture depicts the targets of an archer. The blue clustered targets show that the archer is way-off from the actual target (in our case, measurement) but is consistent. However, our measurements need to be close to the target in order to be valid. Therefore, reliability does not guarantee that our device measures fine. We need validity so that we can say that our device measures what we actually want to measure.
