Let's use an analogy: you and I are at opposite ends of a large room. If we want to talk to each other, we're going to have to speak louder if we're farther apart or if others are speaking (otherwise I won't be able to understand what you're saying).
The output power is the power with which your modem is transmitting toward the DSLAM. It's measured in dBm (or the logarithmic ratio of the power relative to one milliwatt), so -22 dBm is about 6µW. For reference, the minimum receive power of a wireless network is usually around -100 dBm (or a tenth of a picowatt).
VDSL2 has a feature called UPBO (Upstream Power Back-Off), which means that the equipment on both ends will transmit with the minimal amount of power to get a good SNR. Transmitting with higher power leads to more crosstalk between your copper pair and your neighbours'.
Since you're very close to the DSLAM (your attenuation is very low, and SNR very high), your modem doesn't need to "yell" as loudly to communicate, so the small number is nothing to worry about!