Lord Shiva is very fond of Abhishek. To make a continuous abhishek on the Shiv Ling, they put a copper pot which a very small hole at the bottom. This way water's abhishek runs for long period of time. Some people also put milk init.
Majority of Shivling worshipped in temples and sacred places have pots hanging on top of the Shivling. A hole is made on the pot and water drips continuously on the Shivling. There is a symbolism attached to the water dripping from the pot. Legend has it that the water in the pot is meant to cool Shiva of the effects of drinking the halahala poison during the Samudra Manthan.
Symbolically the water dripping from the pot indicates that life is always about movement. There is no stagnation. If we become stagnant then we rot and this is against nature.
Life also ebbs out slowly like the water dripping from the pot. The short period that is available should be used to attain self realization.
Each moment in our life is like the water drop, we should dedicate it to the supreme truth. Perform our dharma by submitting all actions to the supreme truth.