What does iPhone assistive touch mean?

I fell in love with a used iPhone 4 on Amazon and the description says the on/off button doesn't work but you can use assistive touch to turn it on. What does this mean and if the phone is off, how do I change the setting to assistive touch?

You plug it into charging and the device will turn on. And then you can change those from
Settings > General > Accessibility > AssistiveTouch > Slide the switch to On
but then again if you switch the phone off or display turns off after some time ( again can be changed from settings menu) you have to again plug it in a charger.

The Assistive touch stays on as long as the screen is on.

out of context but i suggest you don't buy it or if you do. Consider getting the button replaced. Obv. The economics have to be considered.