Soft PLA is another name for flexible PLA. It is a natural 3D printing filament although it not exactly like PLA but it's quite similar and biodegradable.
It also has excellent inter-layer adhesion, so the prints are extremely durable. Think of it as soft Nylon, as it shares a few similar properties. One of them being that flexible PLA has low friction coefficient.
It is a versatile filament; you can print it thick walled or with more density of infill for a stiffer more durable print. Or you can print it thin walled with lower infill, so it's more soft and flexible.
It is a great starting material for beginners to get familiar with flexible prints before moving on to move advanced materials such as TPU.
The main difference between a standard and flexible PLA filament is slower printing speed, and higher temperature bed is required for flexible PLA.