What does this equation mean?

Bandwidth in particular has a hidden physical driver that explains why these four approaches look so different from one another: pin count. A memory bus’s raw bandwidth is well approximated by $B \;\approx\; W \cdot f_{\mathrm{eff}}$. with W the bus width in bits and $f_{\mathrm{eff}}$ the effective transfer rate per pin. For a conventional DIMM interface, W is set by the number of package pins a processor can spare and a printed-circuit board can route, and that number has a hard physical ceiling — DDR5 alone specifies 288 pins per channel, and adding channels for more bandwidth becomes markedly more expensive under a CPU package’s pin budget [ 6 ] . Once W is capped by the board, the only lever left is $f_{\mathrm{eff}}$ , and pushing per-pin speed higher costs power and signal-integrity margin at an increasing rate.