Think about what this code will do if value is < 100.

while (value < 100);
        System.out.println("thank you" + value);

It will repeatedly print: "thank you" + value. In other words, it will be an endless loop.

The example code 

while (value >= 100) {
        System.out.prin("Please enter a value less than 100");
        value = scanner.nextInt();
}

will remain in the loop while the user enters unacceptable numbers. The loop terminates when the user enters a number that is < 100.

This may seem somewhat backwards. The loop repeats while something is wrong. Normally you think of a loop repeating to accomplish something positive. This example uses loops in a negative way. But it is correct.