Tracing is the act of using a photo or a picture and copying. You are copying lines very precise by overlapping paper-to-paper and tracing the lines in traditional mediums. In digital art you often have different layers, and you copy lines from the layer underneath. It is often easier if you have the layer underneath in a lower Opacity.
Some people might see tracing as cheating. What is important is your own enjoyment and your own opinion, you should not listen to what others say if you are having fun.
On Art, we allow tracing as long as you credit the original artist. Stealing other peoples art and stealing credit is a community guideline violation.