Lets take a look at the Viniyoga Shloka (uses)
viniyogastu tasyeva shivalingastya darshane
The shloka say that this Mudra indicates a Linga of lord Shiva. Linga means sign, mark or symbol. Thus a Shivlinga is a symbol that represents Hindu God lord Shiva.
Linga is derived from the Sanskrit word lingam, which comes from li meaning, “to dissolve” and gam “to move on”. This refers to the belief that one appears as a being in the world and then dissolves back into the universe. A Shivalinga is phallus-shaped and fixed on a base, which is shaped like a yoni. The structure symbolises the supreme creative energy. It represents the Shivashakti ie the union of Lord Shiva and Devi Parvati. It is usually made of stone.