ELI5: You've got DC+ and DC- coming in, and AC1 and AC2 going out. Hook up + to AC1 and - to AC2, power goes forward, a crest. Now switch the cables quickly, - to AC1 and + to AC2. Now power is going backwards, a trough. Keep doing that 60 times a second and you've become a simple square wave inverter. Replace you with some circuitry and it's automatic. Normally you use more complicated components to generate a sinusoidal wave but the gist is the same, keep switching the DC input forwards and backwards and it creates the AC voltage wave