Facts are the meat of a family tree. The names and dates. When a person was born or died. Marriage dates or dates they appeared in a census. Family trees all over the internet have all sorts of facts. But you can't just take them at face value simply because it's on the internet or because the website 'looks' trustworthy.
The facts may not be backed up by sources. Or the sources used may not be credible or reputable. You need to become good at evaluating not only the facts you find, but the validity of them.
I strive to make sure the information in my tree all has a source attached to it. Without a source, that information cannot be verified as accurate. It becomes unproven, legend, or family lore and doomed to be repeated across the internet forever. Let's try to break the cycle of bad information in genealogy.
I'm not perfect. Sometimes I'll add a fact to my tree, intending to go back and add the source and then I forget. If you find a fact without a source, please let me know.
This is not a complete list, and I'm sure I'll add to it as I think of more.