The graph shows the strength of a WiFi radio signal (green) received at my computer. It also shows the background noise (red) my computer detected. A radio link will be better if there is a strong signal and little noise, if there is a high signal to noise ratio (SNR).
In this example, I started with my laptop in my office, the same room as the base station it was communicating with. After one minute, I picked the computer up and walked into the living room. As you see, the signal was attenuated (reduced) since it now had to travel around 25 feet and go through an interior wall, a large, leafy plant and some furniture.
The unit of measure for the signals is dBm which stands for decibels below one milliwatt. When my computer was in the same room as the base station, the signal was fairly steady at around -33 dBm and when I settled down in the living room it was down to around -57 dBm. While I was walking and repositioning the computer, it dropped to around -67 dBm.
Decibels are a measure of the ratio of two power levels. As a rule of thumb, power is doubled with each three decibel gain and halved with each three decibel loss. The signal power level in my office was about 24 dBm above that in the living room. In other words, it was around 256 times as powerful (eight doublings).