Below is a quick info sheet for ADSL when it comes to those Signal to Noise Ratio or SNR and Attenuation numbers usually found on ADSL routers:
Signal to Noise - The higher the better, this indicates the ratio of signal power to noise power, ie. the background noise should be as low possible and the signal power as close to desired as possible.
SNR can be influenced by various factors, these include weather conditions and sources of interference eg. electrical cables running over a phone line.
6dB - No sync or intermittent sync
7-10dB - Fair service level
11dB-20dB - Good service level
20dB-28dB+ - Excellent service level
Attenuation is the gradual loss of signal over a distance, this occurs due to the resistance of the copper wire being used and various other telecom goodies between you and the exchange. The lower the attenuation the better and the more likely you become to sync at at higher data rates.
The graphs below give a good indication of the influence of both distance and attenuation will have on the average ADSL, ADSL2, and ADSL2+ line: