What is the difference between V_out and 3.3?

I think @allObjects covered this nicely, but the naming on the pinout page at https://www.espruino.com/Pico#pinout makes this less confusing than the board's markings...

• V_IN / BAT_IN is where you'd connect a battery - the Pico will use the battery power when USB is disconnected, and when USB is connected it'll use that.
  
• V_out / VBAT is the output voltage. It's USB power (minus the diode voltage drop) when USB is connected, or when it's disconnected it is the battery power from V_IN / BAT_IN