Mary had stepped close to the robin, and suddenly the gust of wind swung aside some loose ivy trails, and more suddenly still she jumped toward it and caught it in her hand.

This she did because she had seen something under it--a round knob which had been covered by the leaves hanging over it.

It was the knob of a door.
She put her hands under the leaves and began to pull and push them aside.

Thick as the ivy hung, it nearly all was a loose and swinging curtain, though some had crept over wood and iron.

Mary's heart began to thump and her hands to shake a little in her delight and excitement.
The robin kept singing and twittering away and tilting his head on one side, as if he were as excited as she was.