IC 4628 is a large and faint emission nebula, of about 250 light-years across, located some 6,000 light-years away from Earth in the southern constellation of Scorpius (the Scorpion).
The nebula is a huge region filled with gas and clumps of dark dust. These gas clouds are star-forming regions, producing brilliant hot young stars. In visible light, these stars appear as a blue-white colour, but they also emit intense radiation in other parts of the spectrum, most of them in ultroviolet. That is the reason we can see a blueish color on the top left side of the image.
It is this ultraviolet light from the stars that causes the gas clouds to glow. This radiation strips electrons from hydrogen atoms, which then later recombine and release energy in the form of light. IC 4628 is an example of an HII region (ionised hydrogen).
We can also see dark nebulae strucutures all over the image. The big one in the middle of the image is catalogued as Bernard-48 and seems to be connecting both hemispheres of the nebula

This image was acquired in 2 days, according to the following stack:
Luminance: 15x20 minutes
RGB: 26x10 minutes each channel
All bin 1x1

Scope: Skywatcher Esprit 120mmf/7
Camera: ST-8300M
Mount: Skywatcher Eq-6 Pro
CCD Guider: Lodestar
Capture Software: Maxim DL
Pre and Post-Processing: PixInsight