Assuming you know the meaning and usage of decibels well:
That image + the accompanying text in the linked source tries to warn that high dynamic range (=the dB difference between the noise floor and the maximum undistorted input signal level) isn't alone useful information.
I'm not sure where the numbers are taken to the image - they can be measurement results or taken from datasheets. But the possible slight inaccuracy of the numbers doesn't spoil the purpose of the image.
PMD671 seems to have inferior dynamic range when used to record a mic signal - only 65dB. One may wonder that how in the hell PMD671 can make anything useful with a seemingly superior mic, say MKH60? Its dynamic range is whopping 119dB.
The image is drawn to tell that it's useless to have a mic which maybe can tolerate close miking a heavy metal drummer or a jet with no distortion if one wants to record quiet sounds with no hiss. For ex. MKH60 has remarkably higher self-noise output voltage (=-114dBu) than what's the noise floor of the mic preamp of PMD671 presented as noise input voltage (-121dBu) of a noiseless amp. The additional noise caused by the mic preamp of PMD671 is only +0,8dB to the noise floor of mic MKH60.
The calculation with the triangle is in accordance how the R.M.S voltages of non-dependent noises should be added together. It's easily proven in the elementary math of communication engineering.
The image also shows that the noise floor of the line input is nearly as low as needed to fully utilize the common 16 bit integer coding of digital audio. 24 bit coding is is quite useless because the -100dBu noise floor of the line input is much higher than how weak signals would be in theory possible to be coded with 24 bits.
One interesting thing (maybe the most interesting one) is NOT shown in this image. You may know the SOUND level of a weak interesting voice, music, etc... as unweighted decibels over the hearing treshold or micropascals (=sound pressure) or in frequency weighted dBA sound level units. You may want to know what's the reachable signal to noise ratio when one uses say MKH60 mic and PMD671 to record it.
To get it you must calculate the signal voltage the mic generates. You simply multiply the mic sensitivity (plain volts per pascal, not dBu/Pa) with the sound pressure in pascals. Then you must compare that voltage in decibels to the noise floor R.M.S voltage which is already calculated to be 1,68 microvolts with that mic and recorder combination.