A wheel’s bolt pattern is a notation indicating the number of bolts and their layout on the wheel.  For example, 655 BC (or sometimes 6x5.5 or 6x5.5”) indicates 6 bolts on a 5.5” diameter circle (an imaginary circle through the center of each bolt hole.  There are several bolt patterns so make sure you find the right wheel to fit on your axle.  Other common bolt patterns for utility trailers include 545 BC and 865 BC.