The lower 16K of RAM in 48K (and 16K) ZX Spectrum computers is provided by eight 4116 DRAM ICs. It is common for these to fail through simple old age or due to other faults in the computer, in particular failure of the circuit that generates the multiple power supply voltages needed by the 4116s. It is also quite likely that at least some of the 4116s will be damaged should a device be connected to or unplugged from the Spectrum's expansion connector while the power is turned on or if such a device is improperly plugged in (something that is very easy to do). New old stock and used 4116s are available, but they are probably at least thirty years old and there is little reason to think they will be more reliable than those they are used to replace.
The ZX Spectrum lower RAM replacement module is a device that replaces all eight 4116s in a 48K or 16K ZX Spectrum (it cannot be used with any of the 128K models). It uses a modern SRAM IC and is far less likely to be damaged in the situations described above. Another, smaller, advantage is that it consumes less power than the 4116s and so the heat generated inside the computer is reduced, mainly due to the lower current through the 7805 voltage regulator. The module can be inserted into standard DIL sockets (it is not a good fit for turned-pin style sockets) fitted in place of the 4116s or it can be soldered directly to a Spectrum's circuit board, once the 4116s have been carefully removed. No other alterations need to be made to the computer. From the point of view of software running on the ZX Spectrum, the computer's operation is unaltered by the presence of the module and it should not be detectable.
Compatibility note: It is anticipated a ZX Spectrum with a 5C102E ULA will not function properly if fitted with this module. Feedback suggests the same problem affects not more than 1% of ZX Spectrums that have a 5C112E ULA. No later versions of the ULA are known to be in any way incompatible. In cases where this incompatibility occurs, corruption of the display is apparent, but the computer otherwise functions normally. The problem might be cured by replacing resistors R17 - R23 with ones of slightly higher resistance, but this has not been tested. Note that exactly the same symptoms can occur because of a faulty ULA, regardless of the type of lower RAM installed. No such compatibility problems are known the affect the upper RAM module.