The Pual stem represents intensive action in the passive voice (as opposed to the piel which is intensive action in the active voice). See more here or this video.
The fundamental idea of Piel [and the Pual], to which all the various shades of meaning in this conjugation may be referred, is to busy oneself eagerly with the action indicated by the stem. This intensifying of the idea of the stem, which is outwardly expressed by the strengthening of the second radical, appears in individual cases as
(a) a strengthening and repetition of the action;
(b) a causative sense (like Hifil);
(c) Denominatives are frequently formed in this conjugation. (source)
