The basic difference
A primary source is the original material: a court judgment, a company's accounts, a government statistic, a recording, a scientific paper, or the words of someone who was there. A secondary source is someone describing or interpreting that material. Most news stories are secondary, and that is fine, as long as you know it.
Why the chain matters
Information passes through hands. A study is summarised by a press office, then by a news agency, then by a website, and each step can lose a caveat or add an exaggeration. By the time you read it, a careful finding that 'may be linked' can have become a confident claim that something 'causes' something else.
How to find the original
Good articles name or link their sources. Look for phrases like 'according to a report by' or 'documents seen by', and then search for the report itself. Many official figures, court documents and studies are published free online. Even reading just the summary page of a report can show whether a story has represented it fairly.
Anonymous sources
Sometimes journalists rely on people who cannot be named, such as whistleblowers or officials. This is legitimate and has exposed real wrongdoing, but it asks you to trust the outlet's judgment. Check whether the story gives a reason for the anonymity, whether more than one source confirms it, and whether the outlet has a record of getting such stories right.
Circular reporting
A common trap is when outlet A reports something, outlet B repeats it citing A, and outlet C cites B. It looks like three confirmations but it is really one claim. When several stories use identical wording, ask whether they all trace back to a single original.
A simple habit
For any story that matters to you, ask one question: where did this come from? If you can follow it back to something you can read yourself, you are on solid ground. If you cannot, hold the claim more loosely.