The Mouse object allows projects to respond to mouse input.
When designing your project, you cannot assume everyone has a mouse. Many users browse the web with touch-screen devices that have no mouse. Therefore if your project uses exclusively mouse or keyboard control, it may be impossible to use on touch devices. See the Touch controls tutorial for an alternative control system.
If you only use left clicks, consider instead using the Touch object with Use Mouse Input enabled. This will allow your project to work on touchscreen devices without any further changes.