The signal-to-noise ratio (SNR) compares the level of the Wi-FI signal to the level of background noise. A ratio of 10-15dB is considered unreliable and a ratio of 16-24dB is considered poor.
First, you should try to determine whether your problem is with your signal strength or noise level. Take a look at your Signal Level and Noise Level visualization heat maps. Check the color in the map areas that had registered low SNR. If you are seeing blue or purple on just one of those visualizations, then that is where your main problem lies. If you are seeing blue or purple on both visualizations, then you have issues with both signal strength and noise.
Next, deselect all of your APs on the left sidebar and then re-select them one by one. Try to narrow down which APs are affected.
To solve your problem, you will need to either increase the signal strength or decrease the noise (or do both). Please see the following articles for more specifics on each strategy:
Note that identifying and removing the source of noise might not be an easy task. In practice, the easiest solution is usually increasing the signal level rather than decreasing the noise level.