The HP LaserJet M426fdn/M427fdw (often grouped as HP LJ M426/M427) is a series of multifunction printers (MFPs) from HP, which support features like printing, scanning, and copying. The HP LJ M426/M427 Scan Driver is a piece of software that enables the scanning functionality of these printers on Windows computers.

In other words, it's the software that allows your Windows PC to communicate with the scanner inside the M426fdn or M427fdw printer and initiate scanning jobs, such as scanning documents to your computer. This driver is necessary if you want to use the scanner from within applications like Windows Fax and Scan or third-party programs like Adobe Acrobat.