'Understanding' in the Bible

For if I pray in a tongue, my spirit prayeth, but my understanding is unfruitful.

What is it then? I will pray with the spirit, and I will pray with the understanding also: I will sing with the spirit, and I will sing with the understanding also.

howbeit in the church I had rather speak five words with my understanding, that I might instruct others also, than ten thousand words in a tongue.