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: imagine you're at a conference where a charismatic speaker lays out a 2-year prediction of AI, or the stock market, or a specific industry. Most likely the prediction will be wrong, so you don't gain accurate information about the measured, though you do gain information about the measurer (the speaker) - for example, you can tell they are very optimistic about the growth of an industry. Indeed, this is often the only kind of information you can extract from predictions, or most conference talks.
There are many more concepts in my "Mind Expander" tool (it's free)