No original research — example
Let's say you're writing about a nearby river, and find a government report from 2003 shows that Arghtron Industries, which had a factory nearby, was fined $6 million for dumping hazardous waste into the river. Let's say you also find an ecology journal that notes the river used to be home to an endangered species of frogs, which went extinct in 2003.
You might be tempted to write something like, "The pollution from Arghtron Industries contributed to the extinction of frogs in the river." But that's original research. You've fused together two ideas (the pollution, and the extinction) and tied them together. Unless another source confirms the link, you can't make that connection on Wikipedia.
Don't make assumptions of facts, or draw conclusions, on Wikipedia. Those conclusions might seem obvious to you, but you can never know what you don't know. So, only write what you can explicitly verify.