Buddhism Question for those who've been practicing & possibly teaching Insight Meditation for at least 5 yrs

In Ajahn Brahmavamso's: the Quality of Mindfulness
http://www.bswa.org/modules/icontent/index.php?page=80
he speaks of The Gatekeeper and instructing The Gatekeeper
within us during Meditation.

Who or What exactly is the Gatekeeper? Is it simply memory within the Mind? It isn't an entity guiding us is it? Wouldn't necessarily think so, with proper understanding of Buddhism and Mindfulness (Insight) Meditation. Does it actually use the term 'The Gatekeeper' in any of The Buddha's teachings or

Mindful Meditation