On-center spacing, also known as center-to-center distance (and OC or CTC for short), is a method of measurement that describes distance between parallel construction elements based on the center of the material. If a blueprint calls for OC spacing of pieces such as beams, rafters or columns, place the center (not the edge) of each plank or bar in line with the measurement described.
Architectural plans often use OC spacing because the dimensions of building materials typically vary slightly between pieces. If some are a little wider than others, on-center spacing will keep your construction more uniform than it could be if it were planned based on edges.
One specific application of on-center spacing is ensuring support for any wall materials that will be applied in sheets, like drywall. Referring to OC spacing is a good way for building planners to make sure the edges of those sheets will line up with solid backing from a stud underneath.