The garland (mala) of skulls (kapala) that Shiva wears, also called Kapalamala.
Shiva as the dissolving principle, as the lord of time, wears the skull as a symbol of the transitoriness. After every kalpa one is added. They are the skulls of Parvati, which he collected after each of her deaths, strung on the string of the immortal soul.
The number 108 is also mentioned, Shiva does not have any more skulls, in other words, one is not born more than 108 times. The 108th birth leads to liberation.