A more thorough explanation:

Definition: In patent law, when one device contains substantially identical features of another device, it is said to “read on” that device.

For example, in the case of Fellowes, Inc. v. Michilin Prospecity Co., the court found that paper shredders infringed a patent which referred to a “spacer” that “consists of a V-shaped notch,” where the potentially infringing device had spacers, spaced apart cutting disks, circumferentially spaced teeth, and parallel cutting cylinders. The potentially infringing device read on the patent because the spacer could be separate from, or integral with, the cutting disk as long as it was positioned between two adjacent cutting disks.

A court may find that a device which reads on a patented device infringed that patent. Astra-Sjuco v. U.S. Intern. Trade Com’n lays out the two-step process that courts apply in adjudicating patent infringement claims: first, the meaning of the claims in issue must be determined by a study of all relevant patent documents; second, the claims must be read on the accused structures. That is, the allegedly infringing device must read on the existing patent to infringe by essentially being substantially identical.