TLDR; Favor explanations that have the fewest assumptions.
Occam's Razor is a general heuristic used in science to prioritize when there are more than one competing hypotheses. The name comes from an English friar who apparently used the heuristic so often and with such effectiveness that it became known as his "razor".
The basic idea is that we should favor explanations that have fewer assumptions than more complex explanations that involve many assumptions. Conspiracy theories are notoriously guilty of these kind of assumptions and complexity. The logical soundness of a good conspiracy theory can be enticing until you use Occam's Razor to compare with competing explanations. The fewer assumptions that you could possibly be wrong about, the more likely you are to be holding an accurate position.
We can illustrate this with simple coin flips. Let's say we have two competing explanations and the first one has 1 assumption while the second one has 3 assumptions. We'll also assume that each assumption bears a 50% chance likelihood of being wrong (heads you're right, tails you're wrong). This is an oversimplified example just to demonstrate the concept.
With 1 assumption you have a 50% chance of the coin flip landing favorably for you.
With 3 assumptions you have a 12.5% chance of the coins landing favorably for you.
If your intentions are to predict the correct explanation, then the smart gambler would pick the one with the fewest assumptions.
William of Ockham Image credit https://commons.wikimedia.org/wiki/File:William_of_Ockham.png