MSS is an advanced software synthesizer for Linux, i.e it is a program that can synthesize very complex waveforms and play them through the sound cards D/A converter (for more info about hardware and software requirements, see System requirements). It is a real time synthesizer that can be played either via the builtin keyboard (a GUI widget that is played via the mouse), or via an external MIDI keyboard. The "real time" means that any adjustments you do while playing (e.g change of filter settings) are immediately audible.
MSS is a modular synthesizer, i.e the synthesizer module configuration (in terms of the number and type of synthesizer modules, and how they are interconnected) is not fixed. MSS includes a set of synthesizer module types like different kinds of oscillators, filters, envelope generators and so forth (the available module types are described in Module reference ). The synthesizer configuration can in principle be any set of module instances selected from the available module types. New module instances may be added and interconnected via menu choices. It is possible to setup very complex configurations in this way. The limiting factor is in practise probably the available computer resources (CPU speed), and maybe your own imagination in creating complex configurations.
MSS is just a synthesizer. It does not provide any sequencing or recording functionality at all, i.e you can only play it yourself and you can't save your performance. It is simply just an instrument.