What are shaders

A shader is a set of instructions to the GPU, which tells it how to draw a given object. Shaders are paramaterised, which means that you can use the same shader with a number of different textures / colours, with minimal effort. Unity comes packaged with a decent set of shaders, but eventually the time will come when you will need to do something which they don't cover. When that time comes, come here with a specific question, and maybe one of the 5 people or so who know anything about shaders at all will be able to help you!

You do realise you could have found out all of this quicker and more easily by typing your question into Google, don't you? It's in the Unity Reference Manual, as well, which, by the way, is a good read and pretty much vital for you to have a good understanding of the contents of.