'Understanding' in the Bible

For if I pray in an unknown tongue, my spirit prays, but my understanding is barren.

How then does the matter stand? I will pray in spirit, and I will pray with my understanding also. I will praise God in spirit, and I will praise Him with my understanding also.

but in the Church I would rather speak five words with my understanding--so as to instruct others also--than ten thousand words in an unknown tongue.