All em waves, whether light, UV, radio, gamma, etc- can be measured in frequency - so yes, I see why one person is confused- but no, it's just measuring them all in THZ waves instead of nanometers or HZ, but the scale of the spectrum gets big, so they chose that one arbitrarily it seems./
And yes, the graphic mentioning sound- I am not sure what that chosen graphic is actually getting at...I suppose it's more about frequency.
Regarding the Terahertz part of the spectrum- it's neat, in that it's really the area after far infrared, well, blending from that- into Microwave.
So, it's got a longer wavelength, than far infrared, and can penetrate further into materials in general(except some materials which are opaque, of course )- and less energy..../
I find it neat because the deeper you go into the spectrum from an imaging standpoint(something they don't talk about here)- you start to see some unique effects- Example; in a thermal image, people glow because they emit, mostly in the LWIR thermal band(not MWIR, though they glow there too) - as you go deeper and deeper ,they don't emit as much, but they still do- meanwhile, the penetration against waver vapor, gets better and better the deeper you go- so ultimately, you could build imagers that see even farther through bad weather, but still give a unique view of the world with it's own unique properties. We're actually talking about superman-level vision, i suppose.
There's a LOT of other uses for accessing more of the spectrum, but the terahertz, and passive millimeter wave- and further, isn't used in as many applications as it could be yet, due to some problems that have never been bothered to really have been addressed, like the increasing wavelength(not insurmountable) and the chips and sensors required to access those bands...
Example: Driverless cars with access to sensors across more of the spectrum, from long wave infrared ,to longer- would allow passively seeing through rain and fog and obscurants to a better and better degree- and while active sensors are good, passive ones 100% won't interfere with anything else.
Medical imaging- how many conditions of the human body cna be seen, if you just keep going up and down the scale in the right wavelength? Thermal imaging already sees MANY medical conditions on people (I can also confirm this having seen...things like Reynaulds Syndrome on people , just walking around with my thermal cameras in public to my surprise(It was said people who noticed it and pointed it out to me when looking at my thermal cameras) ) - Xray wavelengths don't necessarily see everything - there's a whole lot of spectrum aside fromthem
I leave any fellow geeks who love seeing this stuff, with these images from around the web- To give an idea of what the world looks like as you go further in the spectrum.
http://www.vision4thefuture.org/s4_resources/2_passive.htm [throwing in an Image of the world Further down the spectrum at the passive millimeter-wave area, to be exact, 90 GHZ) https://www.researchgate.net/figure/Photographs-and-correspo...
[Radio Wave imaging of satellites in the sky at 10-12 ghz , couldn't resist throwing this one in- sadly it appears No ground level imaging has been done in these bands, but it would take effort to set up a radio-wave imager, it'd also be a bit large and unwieldy- though still useful} https://www.orbiter-forum.com/threads/the-amateur-radio-astr...