That means that it uses 10 bits per color.
That means it uses 10 bits of data to identify how much red is in each pixel, 10 bits of data to identify how much blue is in each pixel, and 10 bits of data to identify how much green is in each pixel.
10 bits of data allows for 2^10 (1024) different levels of red, green, and blue.
1024 levels of each of the primary colors allows for 1024×1024×1024 = 1,073,741,824 different possible unique colors.
By comparison 8 bits per color is the most common color depth for a monitor to support. If you do the math with that you end up with “only” 16,777,216 possible colors that can be communicated between the computer and the display.
16.8 million compared to 1.07 billion might sound like a huge difference but when at that level the difference is barely perceptible because 16.8 million is already a lot. By comparison most streaming services use only 6 bits per color (262,144) and GIF files are typically limited to 256 possible unique colors.
The problem is that that hub has to share bandwidth with the display. In High-Speed Data mode it shares bandwidth equally, in High Resolution mode it allocates much more bandwidth to the display (allowing it to reach the full resolution, refresh rate, and color depth).
In High Resolution mode it leaves only about 480 Mbps available for the hub. That’s fine for a mouse and keyboard. It’s also okay for the average internet connection, although if you have a particularly fast connection or want to access stuff within your local network that will be limiting, and you won’t get the full speed of any USB 3.0 devices you have.
3440x1440p 100 Hz could potentially be achieved simultaneously with High-Speed Data mode if it is dropped to 8 bpc (16.8 million unique colors), 4:2:2 subsampling is enabled, and reduced blanking V2 is set.
4:2:2 chroma subsampling is essentially a compression algorithm that works by having the computer black and white data for every pixel but color data only for every other pixel. Because humans are more sensitive to changes in brightness than changes in color that results in the monitor displaying something that looks almost as good as if it had color data for all pixels (the downsides of chroma subsampling are most noticeable around text).
Blanking is bandwidth that is deliberately wasted to improve compatibility with old displays. Reduced blanking V2 is a configuration that reduces bandwidth wasted to blanking.
With all those combined it would have just barely enough bandwidth to work at 3440x1440p 100 Hz while still having plenty of bandwidth for the hub.