It’s as simple as it says: you get what you measure. The act of measuring in a sense influences the result being measured. A classic example is that of a fisherman and the net. The fisherman concludes that the smallest fish is the size of the net spacing, when in effect his data was confounded by the act of measurement.
be-radically-skeptical of metrics, and remember to make your own judgement on the merits of any such metrics. Also be aware of your own biases, often times, we just see what we want to see. There has to be a need to prove yourself wrong.
you-get-what-you-measure also has a second meaning, which is related to goal-setting. Namely, that you have to measure outcomes to achieve it, else it’s not clear whether something is getting done or not. I use this in 90-day-transformation to set goals for my personal fitness.