Christ gives us a definition of this term in this very verse, though that definition is hidden in translation and in Christian teaching about the Holy Spirit. Though these words were said to the apostles, let us assume that he also addressed them to us today.
First, Jesus starts by saying “When it shows up, that person”, so we know that this spirit is something that shows up at some time. Since the large context is “we aren’t ready now”, the time is when we are ready for it.
In Greek, the word translated as “the spirit” means literally, “the breath”, specifically, “the breath of life”. The word described the animating force of life. This was also the force of “divine inspiration”. (For more on Jesus’s use of words translated as “soul”, “spirit”, “life” and so on, you may want to read this article).
The Greek word translated as “of truth” has a very different meaning than our modern idea of scientific, provable truth. First, it the words has an article, “the”before it, so it isn’t “of truth” but “of the truth”, but the word translated as “truth” means "true reality" as opposed to the surface appearances of things. These are the deeper truth that are revealed to us or inspire us. Once revealed, they are obvious. The Greek word means, literally, “the unhidden”.
So, a direct translation is the “the life force of the unhidden” or “the life force of the revelation” or “the divine inspiration of the revelation”.
What is that? Jesus tells us in the next part of the verse,.
This is the force that “will guide you all into the whole revelation.” The word translated as “guide” (ὁδηγήσει, hodegeo) means to lead someone where they want to go. So this force takes us into the greater insight, the one we seek or need.
However, this force is not an intelligent agent, communicate its own knowledge. This is made clear by the next part of the verse, which translated literally says, “Because he will not announce on his own.” The reason he can speak the truth is that he does not speak on his own, that is, this force does not speak for its own purposes, that is, from its own self-interest. If this was your own inner voice, you couldn’t be certain of the truth because we naturally deceive ourselves, speaking for our own purposes.
This force is a communication channel alone. The word I translated as “announce” above, also has the sense of “repeat” in the rest of the verse. “ Instead, as much as he hears he is going to announce, and, showing up, he is going to report to you.”
So this force is a force of life, the divine inspiration within you. It has the capability of repeating what it learns of the hidden truths of the universe. However, it has to show up within you, Unlike your always present breath of life, this additional breath is a gift, a coming, a dawning of realization and revelation.