Word of Knowledge
8For to one is given through the Spirit the word of wisdom, and to another the word of knowledge, according to the same Spirit; 1 Corinthians 12:8
The word of knowledge seems to be mentioned nowhere else in scripture, specifically as a manifestation of the Spirit. Thus, we can begin with the words themselves, λογος ("word") and γνωσεως ("of knowledge").
λογος ("word") is a common word in Greek, used for many different things. Perhaps the most notable is that of the Son of God Himself, who is called ο λογος ("The Word") and ο λογος του θεου ("The Word of God") by the apostle John (John 1:1-3) (Revelation 19:13). It basically means a subject, matter or even teaching but can also be used to mean a literal word as in a sentence of language. In that way it is an embodiment of meaning, holding truth.
γνωσιν ("Knowledge") is a basic word coming from an ancient Greek verb simply meaning "to know." The heresy of the day for the early Christians was Gnosticism, which basically held that one could ascend spiritually through knowledge. It involved worship of angels and other sub-deities with special words and formulas in order to ascend the ranks of spiritual standing. That word "gnosticism" comes from that Greek word meaning knowledge.
The Holy Spirit chooses to manifest "a word of knowledge" in His people, for the profit of all. Combined, this manifestation is a capsule of knowledge given to a person that they did not have before. Because it is not limited here by Paul, it would seem to include anything that can be known.
Examples in Scripture
In the Bible, we see a few examples of God's people suddenly having a knowledge of something that they didn't before, without any discernable natural method of them gaining it: Christ knowing where Nathaniel had been before He met him (John 1:47-50); Christ knowing the woman at the well had separated from five husbands and was living out of marriage with her current man (John 4:16-18); Peter knowing Ananias and Sapphira had lied about the amount for which they sold their land (Acts 5:1-10); and Paul knowing his and his captors' voyage would end in shipwreck (Acts 27:10-22).
Paul elsewhere admits that even with the insight of the Spirit, we still see dimly compared to when we receive our new bodies from Yahweh when Christ returns. However, in the meantime, this wonderful manifestation enables us to receive knowledge of the things that are, when we cannot discern them naturally.
The Way to Walk
This manifestation reinforces the truth that it is impossible to discern rightly according to the flesh, according to outward appearance. It's only by the Holy Spirit that we can discern rightly.
24Don’t judge according to appearance, but judge righteous judgment.” John 7:24
When we find ourselves in situations where we cannot proceed with the knowledge we have in the natural, which according to the Lord Jesus, is always, we should seek the Holy Spirit for this manifestation of a word of knowledge; then we will come to know Him better as He supplies the information that only Yahweh Elohim holds, and we will be able to act rightly.