About Milliradians
The milliradian (mrad, or mil) is one thousandth of a radian, about 0.0573°. At 1 kilometer it spans 1 meter, which is why it is the angle unit of rifle scopes and artillery sights.
A milliradian measures an angle, but it converts easily to a size: 1 mrad spans 1 meter at 1,000 meters, 2 meters at 2,000 meters and so on. So a target that measures 2 mrad in the scope and is 1.8 meters tall is 900 meters away.
A circle holds 2000π, or about 6,283, true milliradians. That is an awkward number, so most military "mils" are rounded to 6,000 or 6,400 per circle.
Note that there are several variants of Milliradians, namely NATO Mils, USSR Mils, US WW2 Mils and UK Mils. Information on these variants are available from the links on this page.
About UK Milliradians
The UK milliradian is one thousandth of a radian, or 0.0573°, so 6,283 of them fit in a circle. At 1 meter it spans 1mm.
An object that fills 1 milliradian of your view is 1 meter wide at 1km, or 1mm wide at 1 meter. Divide the width by the angle to get the range – a 1.8m person covering 3 milliradians is 600m away.
Degrees don't give you that shortcut: at 1km one degree is about 17.45m wide, which is nobody's idea of a round number.