Big power banks

Power is measured in Watts. Capacity is how long it can deliver power for, in Watt-hours. So a 1440Wh power source will deliver (on paper at least) 1440W for 1 hour, 144W for 10 hours, 1W for 1440 hours, and so on and so forth.

so what are volts and amps then? Asking for a friend.

Current  or Amps is the flow of electricity

Voltage is the force of the electricity.

The lower the voltage, the higher the current for a given wattage .