Bayes' rule (aka Bayes' theorem) is the quantitative law of probability theory governing how to revise probabilistic beliefs in response to observing new evidence.
You may want to start at the Guide or the Fast Intro.
Imagine that, as part of a clinical study, you're being tested for a rare form of cancer, which affects 1 in 10,000 people. You have no reason to believe that you are more or less likely than average to have this form of cancer. You're administered a test which is 99% accurate, both in terms of specificity and sensitivity: It correctly detects the cancer (in patients who have it) 99% of the time, and it incorrectly detects cancer (in patients who don't have it) only 1% of the time. The test results come back positive. What's the chance that you have cancer?
Bayes' rule says that the answer is precisely a 1 in 102 chance, which is a probability a little below 1%. The remarkable thing about this is that there is only one answer: the odds of you having that type of cancer, given the above information, is exactly 1 in 102; no more, no less.
This is one of the key insights of Bayes' rule: Given what you knew, and what you saw, the maximally accurate state of belief for you to be in is completely pinned down. While that belief state is quite difficult to find in practice, we know how to find it in principle. If you want your beliefs to become more accurate as you observe the world, Bayes' rule gives some hints about what you need to do.