Rebirth is a Battle Arena feature that lowers your rank and resets your stats in exchange for Battle Points and useful bonuses. Some have compared it to Ascension. It is not available until you are rank 500, though you should ensure you are suitably prepared before proceeding.
To rebirth, you must be rank 500 and then use a Rebirth Potion (craftable from the Unobtainium recipe in the potions portion of the item crafting menu. Cost is 1,000,000,000 pixels, 50,000,000 crafting material, and 250 unobtainium) in Ye Olde Pub. This will take you to a confirmation screen. Clicking the Rebirth button will start your rebirth.
Rebirthing will unequip all of your equipment. Your rank will reset to 40. You will lose 9200 skill points, but you will keep all earned skill books. Your stats will reset to those of a rank 40 character. Robacon will also reset to rank 40. Any progress towards your current Mission Kommander mission will reset. The rank requirements for areas, skills, and equipment are still apply.
Each rebirth will increase Arena EXP gain by 10%, up to a maximum of 100% during your 10th rebirth. Each rebirth will increase the hardcap on attack, defense, HP, MP, accuracy, and evasion by 1%, up to a maximum of 100% during your 100th rebirth. Both of these changes are permanent. You can also gain extra skill points as usual. Each time you rebirth, you earn 2040 Battle Points. During a rebirth, you earn extra Anti-Idle: The Game Experience, as follows: