pali word:- avijjā, opposite is:- vijjā.
vijjā = transparent.
avijjā = non transparent.
puthujjana mind is avijjā (non transparent), because it is clouded by 5 hindrances (nivaraṇa), that make it opaque. so the sati (mindfulness) is not clear to see what is inside the mind itself.
what is the 5 hindrances (nivaraṇa)?
1. kāmachanda (sensual desire), common is think of sex.
2. byāpāda (ill-will), common is think of hate.
3. thīnamiddha (sloth and torpor), common is sleepy or drowsy.
4. uddhaccakukkucca (restlessness and anxiety), common is the mind is not free.
5. vicikicchā (doubt), common is uncertainty.
to meditate is to focus the sati at one spot, and the mind get away from this 5 hindrances (nivaraṇa).
once your mind can get away from these 5 hindrances (nivaraṇa), the mind turn transparent to the sati.
and you start to have insight into your mind.