What is a "recovery mode"?
Android's recovery mode allows rom-flashing even if the device is currently unusable. It also gives you the opportunity to factory-reset your device, wipe the cache partition, and – given you've got a custom-recovery (such as twrp or clockworkmod) installed, even more.
Invoking it usually involves a key combination that is pressed when starting the device. These combinations differ from device to device, and might also depend on the recovery image used.
There are multiple ways to do this (as the linked article explains). On most devices, it's a 3-step:
- Shut down your device (power off)
- Press the Volume-Down button (and keep it pressed), then simultaneously press and hold the Power button
- Release buttons once the boot sequence is finished
The button combination might be another one on different devices:
- Samsung Galaxy Series: Volume Up + Home + Power
- Galaxy Nexus (Nexus 4, Nexus 7, and Nexus 10): Volume Up + Volume Down + Power
- On some devices With Camera Buttons: Volume Up + Camera
If none of those work, follow up to the linked article for more alternatives.