Unlocking the Bank
When you Rebirth, you lose all cards on your plot and all cash. The Bank system, unlocked at Rebirth 2, allows you to save your best cards. You get 1 slot at Rebirth 2, and slots increase with each subsequent rebirth. Full access is granted at Rebirth 5.
Beginner Rebirth Trap
Do Not Spend Your First Rebirth Too Early! Wait until you have Gold-tier cards and ~$50,000 cash. Rebirthing immediately without a Bank to protect cards is a massive setback.