Noob question. When I hit few times, sword glows purple. why?

[Dysbro]

And what do I have to do when it starts glowing? Can I do like a critical hit something like that?

[⋠Ĝ๏ą✞⋡'ƦэŁĨq.]

Press right mouse button when ur sword is purple, you will then have an empowered attack