Breadcrumb trail
In short: The pieces of bread Hansel drops to mark the way home through the forest.
Object
How does it work in the tale?
Moonlight makes his first trail of stones visible. The same plan fails with bread because birds eat it. The two scenes show how a sign only helps if it stays where it was left.
Why do crumbs fail where stones work?
On the first journey Hansel drops white stones and finds the way by moonlight. On the second he drops bread crumbs, which the birds eat. The plan stays the same so the change in material stands out. A trail in the forest only helps if it stays where you left it.