Cancer is a disease of our own cells. Normally, when our cells become old or damaged they die and are replaced by new, identical cells. For example, when we damage our skin through a cut, new cells must divide to replace those we have lost. Normally this process of division is under tight control. However, cancer occurs when our own cells divide uncontrollably to produce cells that the body does not need, forming a mass of tissue called a tumour.
You might be thinking: what causes cells to start dividing uncontrollably in the first place? The answer lies within mutations. Mutations are changes to a cells genes or DNA. Some genes act as the brakes on cell division i.e. they tell cells to stop dividing. Others act as accelerators, giving cells a ‘go’ signal to start dividing. Cancer is caused by mutations in either of these two types of genes which either keeps the gas on the accelerator or takes the foot off the brakes, leading to cells dividing without control.
Now you might be asking: what causes mutations? Mutations occur naturally when our cells divide and copy their DNA. Mistakes are made randomly during this process and sometimes are not repaired. This is why the incidence of cancer increases with age: because the tissues of older people have undergone many more cell divisions than younger people. However, mutations are also caused by exposure to environmental factors such as tobacco, UV light and certain chemicals. Additionally, some people are born with certain mutations that they have inherited from their parents.
Cancer are malignant tumour (a lump or growth in the body). A lot of people have benign tumours, and many types of benign tumours can be triggered and turn into cancer. Although cells in benign tumours can be dividing very fast and have mutations (like cancer), it is localised and do not move outside where it is. The process of a benign tumour turning into cancer is known as “tumour progression”; in this stage, the fast dividing cells inside the tumour are able to invade outwards and go to other body site (secondary cancer).
In addition to what Sophie already mentioned, what you heard could be referring to whether if the tumours in our body has gained enough mutations so that they can move out of their original site.