The last two WP:60 editions have focused on closely-related policies: "No original research" and "verifiability". A third related rule is the guideline called "Identifying reliable sources", which advises to verify non-original research with "reliable, published sources".
What counts as a reliable source (often abbreviated WP:RS, or just RS)? Unfortunately, there is no simple answer. Among the things Wikipedians look for: what kind of work is it, who wrote it, and who published it? An informal rule of thumb: if it was written by a journalist for a print publication, it probably counts.
But that leaves out much else that might. A technology blog may be reliable within its area of expertise, but not about something unrelated.
Many specific circumstances can affect how a source is viewed, and we'll discuss these eventually. But for now, you have the framework.