The concept you want to understand is metastasis. When a cancer metastasizes, it sends out cells that become cancers elsewhere in your body. One day just just have a little melanoma, the next day it sounds out cells that become cancers in other organs: your lungs, your liver, etc.
Some skin cancers, like basal cell carcinoma, never metastasize. Melanoma is one that will.
https://en.wikipedia.org/wiki/Metastasis