What is the principle of onus probandi?

Excuse me, could you please explain what the principle of onus probandi entails? I understand it has something to do with the burden of proof in legal proceedings, but I'm curious about the specifics. How does it work in practice? Who exactly bears the burden of proof, and under what circumstances? Is there a standard process for determining who has the onus probandi in a given situation? I'd appreciate any clarification you can provide.