Conversion of the real voltage value to the number

Projects Programming
3

in a standard UNO
0V equals 0
and
5V equals 1023

that means every step = 5V/1023 ~4.8...milliVolts

If you want a led light up if the voltage is above 2 Volt you can calculate the analogRead() value expected
2Volt / 4.8... milliVolt = ~ 409

if (analogRead(A0) > 409) digitalWrite(led, HIGH);
else digitalWrite(led, LOW);

or you can work with the voltages in your program

float raw = analogRead(A0);
float voltage = raw * 5.0 / 1023;

if ( voltage > 2.0 ) digitalWrite(led, HIGH);
else digitalWrite(led, LOW);

hope this helps