"The number of ranks on any DIMM is the number of independent sets of DRAMs that can be
accessed for the full data bit‐width of the DIMM ie 64 bits. The ranks cannot be accessed
simultaneously as they share the same datapath. The physical layout of the DRAM chips on the
DIMM itself does not necessarily relate to the number of ranks. Sometimes the layout of all
DRAM on one side of the DIMM PCB versus both sides is referred to as "single‐sided" versus
"double‐sided". These terms may cause confusion as they do not necessarily relate to how the
DIMMs are logically organized or accessed."
"Generally Single Rank Memory is faster than Dual Rank Memory, in laymen’s terms when a
computer accesses Single Rank Memory it only has to go around the track once, where are Dual
Rank it would have to go around the track twice."
from literally the first google result