Oh, didn't realise there's an image provided. I was just replying to the question "how to best read a voltage on A0", and that's exactly what that line of code does.
We're not providing courses on how to use Arduino. You get the answer - but are expected to look up terms/function calls/etc you don't understand. And if you still have questions, feel free to ask.
By the way, that sensor is rather useless. It can't reliably measure levels (if there are more or less minerals in the water your measurements are off. I once tried to turn it into a capacitive sensor (after cutting off the head), even that failed. You'll probably be able to use it to sense the presence of water, e.g. whether a container is full or not.