In plain terms, COF measures how slippery a surface is, and how much friction it generates. More friction is better, because more friction means better traction and thus a lower likelihood of slipping. A low COF means there is less friction, and thus, it is easier to fall.
Jones and Childers report coefficients of friction of about 0.7 for dry roads and 0.4 for wet roads. The tread design represents an "all weather" compromise. If you were an Indianapolis race driver, you would use "slick" racing tires with no tread. On dry surfaces you might get as high as 0.9 as a coefficient of friction, but driving them on wet roads would be dangerous since the wet road coefficient might be as low as 0.1 .