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 rom tag.
Questions (including answers) giving you more details include:
- What is the meaning of “flashing a custom ROM”?
- Why is it that Android still can't be installed as regular OS?
- Do I need to revert to stock firmware and/or wipe data before installing a custom ROM?
- What to do before root and flash my phone?
- How can I keep settings after flashing a ROM?
- Can I install any ROM for my Android device?