The Vánar swift as thought compressed His borrowed bulk of limb and chest, And stood with one quick bound inside The monstrous mouth she opened wide. Hid like the moon when Ráhu draws The orb within his ravening jaws.

In plain words: Quick as a thought, Hanuman shrinks his body and jumps right inside her open mouth. He disappears into it, like the moon swallowed by Rahu, the demon said to cause eclipses.