well perlin normally return a number from -1 to 1.. if yours returns from 0 to 1, then maybe some conversions have been applied to it.

something like (0.5 + 0.5 * perlin value.)

and that line means set a variable with the perlin height number

normaly in this step you would also the above transformation and a height amplification

like (0.5 + 0.5 * perlin value.)* max height value.

Then make a look from 0 to the pelin value, assuming that your height array starts at 0. where 0 value = bottom of the world, some number in between lets say 64 = water level and 128 would be something like the highest mountain value.