For Hume there are two types of truths--matters of fact and relations between ideas. Matters of fact are empirical facts about the world--like that there's a tree in my front yard or that it's 76 degrees right now. Relations between ideas are truths like 2+2=4 and 'all triangles have three sides'. These are not 'about the world' because we don't have to invesitgate the world to know that they are true.

Unobserved matters of fact are empirical truths that either 1) are about the future, or 2) have simply not been observed--like some fact about some planet in some distant reach of the galaxy that we have never seen.