The Reinvest function allows you to automatically buy additional hashrate as soon as your Balance allows it.
Activating this function will use your current Balance to purchase the maximum possible amount of hashrate.
After every payout the system will check if your Balance is sufficient to purchase at least a minimum amount of hashrate (10 GH/s for SHA-256 or 1 MH/s for Scrypt). If it is, then the purchase will be created and automatically confirmed.