What is onus probandum?

Could you please clarify for me what the term 'onus probandum' refers to in the legal context? Is it related to the burden of proof, and if so, how does it specifically impact the legal proceedings? Furthermore, are there any specific scenarios or cases where understanding this concept is particularly crucial? I'm keen to gain a deeper understanding of its implications and how it relates to the determination of facts and outcomes in legal matters.