This icon indicates that the current phone is in fast charge mode. The Xiaomi phone uses Qualcomm Quick Charge3.0 fast charge technology. Qualcomm has announced a new generation of fast charging technology: Quick Charge3.0, which uses an optimal voltage intelligent negotiation algorithm to achieve optimal power transfer at any time and maximize efficiency. It can charge a typical phone from zero to 80% in 35 minutes.
Quick Charge 3.0 comes in 200mV increments and offers a flexible choice of voltages from 3.6V to 20V.
This will allow the phone to get just the right voltage to achieve the desired charging current, minimizing power loss, increasing charging efficiency and improving thermal performance.