"tension" is the wrong word. You mean "voltage" (I hope).

As the light on LDR goes down, the resistance of the LDR increases. This increases the total resistance between across the two resistors, so less current flows (ohm's law) through the two resistors. Since the 10k resistor is still 10k, that means the voltage drop across that resistor is smaller, while the voltage drop across the LDR is larger.

Take a look at Ohm's law - with some basic rearrangement, you can make all the relevant equations. The key thing to remember is that you can assume that no current flows into or out of an arduino pin set as input, so you can set up I = V/R for both resistors - and then they're equal to eachother, so V1/R1 = V2/R2 or V1/V2 = R1/R2, which is of course the basic design equation for a voltage divider.