tasya its conjunction
hetuh cause, ground, purpose
avidya ignorance, lack of awareness, lack of spiritual knowledge
Lack of spiritual understanding (avidyaj is the cause of the false identification of the seer with the seen.
In 11.18, it was said that the amalgamating of prakrti with purusa can either lead to emancipation or block one's progress by enmeshing one within desires and emotions. This sutra emphasises the fact that avidya, ignorance or lack of awareness, is at the root of the confusion that fetches suffering to one as well as pleasure. Vidya (discriminative knowledge) destructs ignorance, because a fire will burn only as long as fuel lasts.
Then again, a question can arise about the exactness of right knowledge. When discernment relegates doubt, pure understanding begins the process of disownment and detachment which releases one from the pinions of possessing and being possessed.