Cells divide in order to create new cells. Mitosis is the process required to create a new cell. A cell that is about to divide is called a mitotic figure.
Dividing cells are found in both normal tissues and in abnormal tissues such as cancers. Because cancers grow a lot faster than normal tissues, they often have many more dividing cells. Mitoses are also present in many reactive (non-cancerous) conditions.
Pathologists can see mitotic figures when they look at tissue under the microscope. Mitotic figures are easy to see because the genetic material inside the cell (the chromatin) changes colour and shape before the cell divides.
A cancer that has many mitoses is growing quickly, but this may also mean it is more susceptible to treatments such as chemotherapy which targets cells that are dividing.