A seat leash is a little wire that connects a quick release bicycle seat to the frame. This prevents someone from walking up to the bicycle, turning the lever, and walking off with that bicycle seat. It doesn’t stop someone who wants that bicycle seat from stealing it – a simple pair of wire cutters will cut right through it, or someone could use a wrench to remove the seat leash – but it prevents casual vandalism.
The term SeatLeashSecurity can be used to describe a fairly low, but non-zero, level of security or WeakEncryption largely meant to keep honest people honest and discourage casual access. Sometimes such an approach is faster and/or more convenient than more robust StrongEncryption. Techniques such as simply compressing data, rotating characters (ROT-13, ROT-39), and XOR scrambling are all examples of SeatLeashSecurity.