A signed driver is a device driver that includes a digital signature. A digital signature is an electronic security mark that can indicate the publisher of the software, as well as whether someone has changed the original contents of the driver package. If a driver has been signed by a publisher that has verified its identity with a certification authority, you can be confident that the driver actually comes from that publisher and hasn't been altered.
Windows will alert you with one of the following messages if a driver is not signed, was signed by a publisher that has not verified its identity with a certification authority, or has been altered since it was released:
This driver either doesn't have a digital signature, or it has been signed with a digital signature that was not verified by a certification authority. You should only install this driver if you obtained it from an original manufacturer's disc or from your system administrator.
This driver was altered after it was digitally signed by a verified publisher. The package may have been altered to include malicious software that could harm your computer or steal information. In rare cases, legitimate publishers do alter driver packages after they have been digitally signed. You should only install an altered driver if you obtained it from an original manufacturer's disc.
A driver that lacks a valid digital signature, or that was altered after it was signed, can't be installed on x64-based versions of Windows. As a result, you will only see this message if you are running an x64-based version of Windows.
If you see any of these messages when attempting to install a driver, you should visit your device manufacturer's support website to obtain a digitally signed driver for your device.