Arcseconds and radians are 2 ways of expressing angular separation... 1arcsecond is 1/3600th of a degree. Astronomers often use arcseconds, in my field we typically use radians... Same thing, just different ways of expressing it.

One arcsecond is about 5microradians... A resolution of a microradian roughly means you could resolve two points separated by a meter at a distance of 1000 kilometers...

They are trying to get giant Radio telescope arrays made up of dishes on the Earth and in space that will have sub micro-arcsecond resolution... Pico-radian resolution... That is like being able to resolve the bumps on a blood cell at 1000 kilometers. Blood cells are ~8 microns across, we are talking resolution equivalent to 1 micron at 1000 kilometers.

They ALREADY have demonstrated 60 micro-arcsec resolution when they took that famous picture of a black hole a few years back...that's approximately the angular size of a baseball on the moon as seen from earth. They want to improve that a couple orders of magnitude.