According to the puranas nandi is considered as the vehicle of Lord Shiva. In general the idol of nandi is located exactly opposite to the shiva lingam which is the idol of Lord Shiva. The idol of nandi is looking at the lord and it seems like he is waiting for the lord to come out to him and tell him something. He is just waiting. In the Indian culture, waiting is considered as the greatest of the virtues and the idol off nandi is a perfect symbol for this. Nandi is an example for eternal waiting.
Those who know about sitting simply and wait are considered as meditative. Mediation needs patience for waiting. Nandi is waiting but not expecting anything from the lord and he will be waiting forever. As nandi is considered as the receptivity essence for the lord, he is the closest accomplice to the lord Shiva. And this nandi will tell you that when you have entered this temple you need to have a quality of sitting simply like him. And he suggesting us that when you have entered the lords place instead of doing some fanciful things and asking the god whatever you need, simply sit like him.
Here the big difference between prayer and meditation is explained. In a prayer we will be trying to talk with the god and tells our needs and in mediation we will be sitting calm and listening to the god what he is saying to us. Mediation shows your willingness to listen to the god who created us and feel his existence. You have to listen to him instead of talking to him. Nandi is not just sitting and sleeping, but he is totally active and alert and he is lively and finally he is just meditating.