[Feba]

S, send, how much data you're transmitting (upstream).
R, how much you're receiving (downstream).

Them hitting 0 for a second or two isn't exactly good, but you shouldn't disconnect unless R stays at 0 for an extended amount of time (which indicates that you have stopped receiving information from the server). This is why the red circle of death (We had it first, dammit, Microsoft always ripping other people off) is often called simply "R0".

They will change wildly depending on what you're doing. Mainly, the more stuff is in the area you're in, the higher R will be. For example, walking around the middle of nowhere, it will be pretty low, walking around a city it'll be higher, the highest is when you're surrounded by mobs (such as if you or someone else nearby has a train of mobs linked together.). Personally, I've gone about as high as 5000-6000 doing that. Really, though, your client is probably going to have problems far before your connection does.

Really, you only need to pay attention to them when the world seems to stop moving. If nobody around you is moving, monsters don't attack and such, you can check it and see if it's at 0, which would be why you aren't getting information. Other than that, it's pretty much useless.
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blech, damn you lack of telepathy.