Ragworts include poisonous substances (though they are not as deadly as some scare stories claim). The main food plant for cinnabar moth caterpillars like the one featured here are those self-same ragworts.
The caterpillars are not harmed by the ragwort poisons. Rather they retain and concentrate these poisons in their bodies. Their defence against predators is to adopt bright warning colours: yellow and black in the caterpillars, scarlet and black in the day-flying adult moth. The message is: eating me will be unpleasant, or worse.