To produce AC from 12 VDC a couple of steps or conversions take place inside the inverter. First step is the input voltage (12 volts) is increased to 145 VDC. Step two, the 145 VDC is converted to 110 VAC by using advanced MOSFET transistors. The resulting AC output from this conversion is called a "modified sine wave". It looks like a squared off sine wave.
Some appliances do not work well or not at all with a modified sine wave. Consult the appliance manufacturer if it is compatible with a modified sine wave output from an inverter. This is especially important for computers and cordless power tools. Equipment not made to support a modified sine wave from an inverter or generator may be damaged if connected to an inverter.