The Sentinels but the well is dry

Her pouting is interrupted by Sam, crashing through the forest blubbering about a 'heart tree' and a 'huldra', neither of which Visala has ever heard of. What? She is quick to respond with. His tone gives the impression that there's reason to panic, but she hasn't heard or smelled anything worth worrying about.

What's a heart tree? She understands the general idea from the name, she thinks she does anyways, but she doesn't understand why Sam is so desperate to make an offering to a tree.