When the leaves fall off the twig, they leave behind a scar where they had originally grown out of the twig. The white scar above is the typical "smile" shaped leaf scar that you see on ash tree twigs, particularly next to the terminal bud. This twig has just started losing the leaves at its tip. You can still see two of the petioles (stems) that connect the remaining leaves to the tip of the twig. Soon the bud will be surrounded only by scars.
Scars also remain on the side of the twig where leaves were attached.
See the new scar there! It's still sort of greenish. Amazing!
And on the other side of the same twig, there's another scar. (Recall that the ash tree has leaves in an opposite arrangement, which means that pairs of leaves grow in opposite directions on opposite sides of the twig, making a \ | / shape.)
Here's one last scar I spotted on a low branch of an ash tree across the street.
This one looks a little darker than the others. Maybe it's older? Maybe the petiole broke off a little differently. This picture also gives you a great look at the large scar that marks where the new growth began this season. That's one ugly scar!