There are two types of pencils - soft pencils and hard pencils. A soft pencil is marked with a B and a hard pencil is marked with H. The soft pencils are soft and smooth to write with and they make thick black line which can make a dirty blurred mark.
The hard pencils make fine lines and do not make the paper dirty. Pressure has to be applied to them if a dark impression is needed.
You can easily make out the difference between a hard pencil and a soft pencil by drawing a chart showing pencils of different hardness.