What does the narrow footpath suggest in reference to Hester's life?

Ah, good question. There is a long tradition in Christianity (and indeed, in other religions) of treating one's passage through life as a journey, and the metaphorical direction one takes as a path. In many of these Christian examples, such as Dante's Divine Comedy, individuals lose their way and/or pass through dark and tangled ways. This is a threat to them, and especially to their souls.

The forest represents wildness, but especially, spiritual threat for Hester, and the narrow path through it represents the narrow path to moral life and salvation.

You'll notice she guides Pearl on the path, and encounters....Dimmesdale.