The term "clicking" can have several meanings depending on the context in which it is used.
1) In a computer mouse, clicking refers to the action of pressing and releasing the button on the mouse, usually generating an audible sound (a 'click') as well as sending a signal to the computer that a command has been executed. This can mean selecting or opening something on the screen.
2) Clicking can also refer to the act of tapping or clicking a finger nail against another surface, creating a sharp and distinctive noise.
3) In psychology, "clicking" is used as a term for an instantaneous change in behavior resulting from a specific stimulus - this could be something positive like suddenly feeling motivated after hearing an inspiring speech or negative such as becoming angry after being insulted.