It means a mature tree in an old-growth forest. Trees that grow in the dense shade of other trees grow slower, and their growth rings are much closer together. The result is that a tree takes a lot longer to grow but it's stronger and harder than the same species grown in the sunlight.

The reason for the distinction is that most of the old growth forests have been clear-cut and the lumber available today is fast-growth farmed lumber. If you compare a 2x4 at Lowe's with a 2x4 out of a 150-year-old house, you'll see that the wood itself is very different even though the species might be the same. The tree the new 2x4 came from was fairly young, while the tree the 150-year-old 2x4 came from was probably centuries old.

Typically it also means one left to grow naturally, without forcing the rate by various methods (as is done in many modern tree farms).

is there any difference to just saying "old tree"?