Ionising radiation includes alpha, beta and gamma radiation and x-rays. It carries enough energy to knock electrons off atoms and make ions. Inside a living cell, this can damage important molecules.
A high dose of radiation can kill cells or damage them badly. This is tissue damage, and it can cause radiation burns and sickness. A lower dose may not kill the cell but can change the DNA inside it. A change in DNA is called a mutation. A cell with mutations may divide out of control, and this can lead to cancer.
Because of these dangers, precautions are needed. Keep the time of exposure short. Keep a large distance from the source, for example by handling it with tongs. Use shielding such as lead or thick concrete to absorb the radiation, and wear badges that record the dose a worker receives.
The danger also depends on the half-life of the source. A source with a short half-life is very active at first, so it is dangerous early on, but it soon decays to a safe level. A source with a long half-life stays radioactive for a very long time, so it must be stored safely for a very long time, as with some radioactive waste.