The trident is the weapon of Siva with which he destroys evils and restores order. The trident stands for the triple qualities of nature, namely, sattva, rajas and tamas. Siva is the master of maya and thereby master of these three qualities with which he controls the worlds. 

The trident symbolically represents the fact that Siva is the controller of the worlds and controller of all illusion. The trident also represents the three aspects of time, the past, the present and the future.

Siva is mater of Time, Kal Bhairav, who determines the progress of the worlds according to his will. The trident represents the three paths to self-realization, namely the path of knowledge, the path of action and the path of devotion. 

With these three paths, Siva transforms his devotees and destroys the evil in them. The trident stands for the three  primary evils which need to destroyed in order to make  progress towards the divine. They are the evil of anger, the evil of lust and the evil of pride.