This is a circuit for a solar night light. It uses a single high bright light white LED running off 2 AA rechargeable batteries.
A solar panel charges the batteries up during the day and the batteries power the LED all night.
So the batteries can be charged fully during the day a simple circuit was added to turn the LED off during the day. When the voltage of the solar panel goes above a certain voltage the transistor turns on which turns off the MOSFET and therefore the LED.
The diode between the solar panel and the batteries stop the current flowing into the solar panel when the solar panel voltage is less than the battery voltage.