If you're measuring your height with a ruler and it shows you're 4 meters tall (13 feet), you gained no useful information about your height. The only information you gained is that the ruler is inaccurate.
Wittgenstein's Ruler is the idea that when you measure something, you are not only measuring the measured (your height), but also the measurer itself (the ruler). You might be getting more information about the measurer than the measured!
Another example: the fact that an employee you know to be brilliant receives a mediocre performance rating doesn’t necessarily mean they are mediocre, it could simply mean the performance rating process is broken.