A softsynth is just what it sounds like it’s a software based synthesizer. Hence the word “soft synth” it’s also known as a VST. Softsynths and VSTs are virtual instruments that reside on your computers hard drive and are played by connecting a midi controller to your computer and accessing them from a vst host software like mainstage or kontakt
Here’s a video that I did a while back that goes into more detail about some of the different softsynths that are out on the market and some of the VST host softwares you can use to play softsynths. Check it out!