Power grids in different countries often have a different voltage, which can be imagined as the pressure in the power grid. European countries usually have a voltage of 220 or 230 volts, whereas the United States have 110 to 120 volt and Japan has 100 volts.
Many devices (including most laptops and smartphones) can operate on either voltage, which means that they can run on both 220v and 110v. In the most cases the voltage range at which the device can operate is printed on the device itself or on its power supply. For devices that can operate at 220v and 110 v you only have to remember that the power plugs are not the same for all countries so don't forget to take the right plug adapter on your travels.
Some devices however only work with a specific voltage. For example, if you would operate a power drill from the USA, which only works with 110v on the German power grid with 230v, the device might get damaged. With a voltage converter you can make sure to safely run devices on a grid with a different voltage.