SNR (Signal to Noise Ratio) is the "Downstream Noise Margin" that is reported by your router. It reflects how strong the Signal is, compared to the background Noise. Essentially it's a figure set by BT's equipment at the exchange, based upon line conditions.
For ADSL1/Max DSL ideally, you would be on a 6dB 'target', which would mean you have a fairly decent quality line. As the BT Exchange equipment detects conditions of instability, it will increase this Noise Margin in 3dB increments which makes your connection more reliable, but at the expense of the actual sync speed. A very rough rule of thumb is that every 3dB increase equates to a drop in sync speed of around 800Kbps.
This holds true unless you are close enough to the exchange to 'cap' your sync speed at 8128k, at which point your sync will stay static and your SNR will increase naturally.
Using an analogy - imagine you and a friend are stood at opposite ends of a room. (you being your DSL equipment, your friend being the exchange). If the room is empty, you should be able to carry on a conversation with your friend at normal volume levels (6dB). If you put a few people in the room who are all talking to each other, you will have to raise the volume of your conversation a bit (a 3dB increase) and sometimes, what your saying may need to be repeated because your friend didn't hear you (800Kbps sync drop).
If the room is totally full of pairs of people all trying to do the same then you end up with a situation where everyone is shouting and no-one can hear what is being said! (bad interference).
However, if your friend comes to stand right next to you, then despite the fact the room is full, you will probably still be able hear what he is saying at lower volume levels, even though the room is fairly busy (8128k capped sync)
