A trail of small marks left to find the way back. Each is meaningless alone; the path exists only in the sequence.
Compound of bread (Old English bread) and crumb (Old English cruma, a small broken-off piece); the navigation sense borrows the image of a trail dropped to find the way back, most familiar from the Hansel and Gretel telling.
A trail of small marks left to retrace a path — each one meaningless by itself, since the path exists only in their sequence.