What radiation does is damage the DNA. basically the building blocks of you. your body has a natural defence mechanism that when your DNA is damaged, the cell is told to stop reproducing and live out its normal life cycle and let other undamaged cells replace it. This is normal, and it's how we combat cancer naturally.
A small radiation dose would damage some of the cells DNA, your body would tell them to stop reproducing and your body would replace them using the good cells around them.
now, as the dose of radiation goes higher more and more cells get damaged. and told not to reproduce. the cells are still there, but they aren't going to reproduce and be replaced. so, as is normal, your cells die but this time as they die they aren't replaced. you slowly start to break down. There is a set time table, certain cells have a certain life span. your stomach is about a weeks turnover, your blood is a couple of days, your skin can be about a month. So, as the cells die normally, nothing replaces them. From the point of a high dose on, you are already dead. it's just your cells are moving around doing their thing. They won't be replaced you won't heal. You are a dead thing walking, talking, and suffering.
Extreme radiation exposure... thats bad, but in a way more merciful? The radiation at this point is ionising radiation. This is strong enough to actually destroy the molecular bond of stuff. So, it's like getting hit with a really tiny machine gun. your cells, and stuff are shot full of little holes as the radiation particles penetrate your body and destroy some of the molecules. this starts to kill the cells.
Think of a cell like a steak. now picture the steak after tenderising it with a spiked mallet.
thats what the ionizing radiation does. couple that with the fact that your DNA is destroyed in the process means that the damage is completely unable to be healed.
With any luck at this point you just die. Because anything else is torture unimaginable.
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