The TTC tells us to look at the Tao even though it can't be seen, to listen to it even though it can't be heard, and to reach for it even though it doesn't have any form.
That all may sound kinda pointless, but maybe what the TTC is saying is that even though the Tao is metaphysical (spiritual, intangible) in nature, we should try to attune ourselves to it.
Now we're told that the Tao isn't bright or dark (which makes sense, since it's invisible).
The Tao is also endless; we can't see its end or beginning.
It is the "form of formless" and the "image of the imageless," which you could interpret as meaning that even though it cannot be seen or felt, it also makes up all things that can be seen or felt (14.13-14).
You could also interpret that a bunch of other ways, so have at it.
The last piece of advice this chapter has for us is that we should use the ancient wisdom of the Tao in our modern lives.