Android recovery is a dedicated partition, with tools that will help you repair/recover your device, clean caches, run updates from SD card etc. You can get into recovery using a key combination specific to your phone manufacturer and model.
Turn your phone off.
Press and hold Vol- and Power at the same time.
When the screen of your Acer Iconia b1-721 flickers release all buttons and wait. In a few seconds the recovery ui will appear.