Why A solar panel produces direct current, but household appliances and the grid run on alternating current. An inverter is the power electronics stage that switches and shapes that DC into a sine-wave AC output at the desired voltage and frequency, typically 120 V at 60 Hz in the US. Grid-tie inverters also synchronize their output phase with the utility line.
Watch out The choice about holding voltage steady as sunlight changes describes a regulator or MPPT charge controller, and the one about preventing discharge in darkness describes the blocking diode that stops current from flowing back into the panel at night.
Invert = flip DC into AC. Diode blocks backflow, controller regulates, inverter converts.