http://www.tirerack.com/wheels/tech/...jsp?techid=101

it's how far your wheels sticks out off the hub basically.
+42mm is as far out as you want on most Accords.

Most aftermarket rims run +42mm-->+47mm.

+35mm -->+40mm will likely stick way to far out.

Stock is somewhere around +50--->+55mm in most cases.