Texture refers to the surface quality of an object. There are two different types of texture you can play around with when drawing any object. However, I think you will find yourself using visual texture rather than creating the actual texture.

Physical texture or actual texture refers to the actual texture you create when drawing. For example charcoal has a totally different texture than something like pastel. You may also be able to create different texture by using different drawing pencils. A 2H pencil will be much finer and smoother than a 9B pencil. So, you may be able to play around with the actual texture, although I myself have never done this. Instead I work with visual texture.

Visual texture is texture you create when you draw something to look rough or smooth. It is an optical illusion because the graphite, charcoal, or whatever medium you are using does not actually have the texture you are creating.

When drawing visual texture it is important to think about how the light is hitting the object. If the object is smooth like glass, light easily bounces of the object. That is why if you look into smooth water you can see your reflection, however when the wind comes along and rough waves begin to form you no longer are able to see your reflection because it is all bent and twisted in the rough texture.

Texture can have an effect on the feeling an artwork conveys. For example, when you have a lot of rough areas in a drawing, the drawing may be visually active and restless. Or, if you have a lot of smooth areas, the drawing may be visually calm and peaceful.

Further Reading

Wikipedia – Texture(visual arts)