Validity and reliability are words that explain how repeatable an experiment's results are.
So for example, imagine an experiment that produced these results from 10 individual experiments;
26cm, 26.5cm, 26cm, 25.5cm, 26cm, 26cm, 26cm, 27cm, 26cm, 26cm.
We can deduce that this particular experiment was reliable, because the results are very similar. However, imagine this scenario;
26cm, 30cm, 23cm, 8cm, 27cm, 26cm, 29.5cm, 33cm, 24cm, 26cm.
These results are not reliable, because they are too different.
If an experiment is reliable, you can follow it's method and expect to get the same, or very similar results.