RWIN (TCP Receive Window) is the amount of data that your computer can accept without acknowledging the sender. If the sender has not received acknowledgment for the first packet it sent, it will stop and wait. If this wait exceeds a certain limit, it may even retransmit. This is how TCP achieves reliable data transfer.
Anyway, this start/stop action slows down the throughput considerably in some cases. Hence, RWIN should be large enough to sustain continuous data transfer. By default, this window is too small (8760 for Windows 95/98/98SE/NT and 17520 for Windows ME/2000/XP) for many types of DSL and Cable (Broadband).
Raising RWIN creates a larger window, which allows more information to come through non-stop, up to a point. After this point, no difference will be noticed for the particular connection. For each user, this point can/will vary and is determined by (bandwidth * delay) product.
Please note that there is no single value for all hosts on the Internet. You have to allocate some reasonable extra RWIN (say 20-25%) for variance from average latency