Flashing is the process of writing a firmware image (and the root file-system) to the memory of an Android device

What is rom-flashing?

Other then installing an OS on your PC, there's no "generic image" fitting all Android devices. Mostly due to proprietary hardware components, each ROM is specific to exactly the device it was created for.

rom-flashing refers to getting those images on your device. For what exactly a ROM is (and related questions), please see the tag.

Questions (including answers) giving you more details include:

Related tags with further information: