The part that is confusing is highest resolution (1) full-disk and (2) outer atmosphere: (1) "Full-disk" is clear to understand: the higher the resolution, ^2 the work to make it also full-disk (especially when the Sun rotates differentially and evolves in high-cadence, so you gotta be fast. (2) "Outer atmosphere" is also tricky as only few wavelengths see the outer atmosphere. The vast majority of the light comes from the "surface" or photosphere (hence the name). In this case surface, the highest resolution is roughly 0.05 arcsec or 50km/pixel. But to see the outer parts, you have to do to emission of elements like Iron that only emit when highly ionized and super high temperatures (those are the special characteristics of the sun's outer atmosphere... yes, it's way hotter than the surface, just WAY less dense). Those emissions happen in the Ultraviolet, 17 nanometers, like the caption says. That's like 50 times smaller wavelength. Angular resolution is proportional to wavelength (1.22*wavelength/Diameter) which is on the order of 1000 km/pixel (but linear resolution makes less sense since the atmosphere is such a 3D shape... it's better to say 1 arcsec of resolution).
I might be too biased (I'm a solar physicist) but the explanation above makes the image way cooler and they should have added it): The most detailed image of the Sun's metal corona :D
You can take an arbitrarily high pixel count photo of anything with enough cameras side by side. But this is exciting (to scientists) because it’s taking the photos in specific wavelength and outside of the Earth’s atmosphere.
The other comment goes into much cooler detail, but if it isn't clear, the reason they needed telescopes in space is because the UV they were detecting can't penetrate the atmosphere. At 17nm you're well into the "vacuum frequencies", just a little away from proper ionizing radiation.
A guy in his backyard takes pictures with similar resolution: https://twitter.com/AJamesMcCarthy/status/147297870519032217... (downscaled for Twitter, approx 9Kx9K / 81Mpix on his Patreon feed)