Bandwidth-delay product (BDP) is a term primarily used in conjunction with TCP to refer to the number of bytes necessary to fill a TCP "path", i.e. it is equal to the maximum number of simultaneous bits in transit between the transmitter and the receiver.
To give a practical example, two nodes communicating over a geostationary satellite link with a round-trip delay time (or round-trip time, RTT) of 0.5 seconds and a bandwidth of 10 Gbit/s can have up to 0.5×10 Gbits, i.e., 5 Gbit of unacknowledged data in flight.
Operating systems and protocols designed as recently as a few years ago when networks were slower were tuned for BDPs of orders of magnitude smaller, with implications for limited achievable performance.
In general, buffer size will need to be scaled proportionally to the amount of data "in flight" at any time.
Because TCP transmits data up to the window size before waiting for the acknowledgements, the full bandwidth of the network may not always get used.
That is because, even after data has been sent on the network, the sending side must hold it in memory until it has been acknowledged as successfully received, just in case it would have to be retransmitted.
If packet loss is so rare that the TCP window becomes regularly fully extended, this formula doesn't apply.
A number of extensions have been made to TCP over the years to increase its performance over fast high-RTT links ("long fat networks" or LFNs).
Path MTU Discovery avoids the need for in-network fragmentation, increasing the performance in the presence of packet loss.
Imagine a Wi-Fi base station having a speed of 20 Mbit/s and an average packet size of 750 byte.