It's a straight forward read a line of input from the console.The input is read as string with readLine function. What is that you do not understand?
A line is as long as till do not press enter in the input.
console.readline is for creating a data entry for the user, ie:
var str = Console.ReadLine()
will bring up an area for the user to type something in, and what this user will type will be stored within the variable str as a string