When we look at the set of SNPs on the Y-Chromosome across individuals, we can see which ones they share in common and which ones differ. And we can use this information to build a tree very similar to the family tree you are all used to, except each node represents an SNP instead of an individual.
Take a look at the tree below. The individuals on the bottom nodes are people from our haplogroup who have taken the Big Y Test, and each node above them represents an SNP. Any individual who tests positive for any SNP on the tree, is positive for that SNP and every SNP on each node above it. You can think of it again in terms of a family tree—if you are descended from your father, then that means you are also descended from your grandfather, your great-grandfather, and your great-great-grandfather, all the way up the line. As you can see from this tree, SNPs below Y6923 have been discovered from these Big Y tests, and these SNPs represent sub-branches of Y6923 whose common paternal ancestors lived as recently as 1100 to 1700 CE.