Buffers are areas of memory where data waits before it gets processed. So a receive buffer holds some data coming from the network until the process (bitcoind) is ready to actually handle it. The send buffer holds data put there by bitcoind until your operating system is ready to actually dump it on the network wire.

They need to be a decent size not to cripple smooth communication. Too large would just be a waste and not help anything. In 99.9% of the cases the defaults should be fine.

They're not a method of affecting the bandwidth use though.

I'm fairly sure the upcoming version will have options for rudimentary bandwidth throttling. Until then you could try lowering maxconnections (default 125) if you want a lower bandwidth on average (over a month for example). But peak usage will still be high.

Depending on your operating system you could try some QOS scripts/settings or QOS settings on your router.