Inverter can either refer to a NOT logic gate, or a power supply that generates AC from DC.

The logic gate inverter turns a 1 into a zero and turns a zero into a 1. On a schematic it looks like a little circle on the input or output of a logic gate or buffer.

The power supply inverter might have been more accurate to call a power oscillator instead. As most commonly used, what it does is it takes a DC source like a battery or solar panel, and repeatedly flips the polarity to make AC. There's usually also a voltage conversion happening.