Don't understand a question on reactions of halogens/halides

A solution of sodium chlorate(l) was added to a colourless solution of potassium
iodide.
Suggest what is observed.
Explain the reaction that leads to this observation (3)

I know that the Iodine causes the solution to become brown but looking at the mark scheme they mention that I- was oxidised and that the brown colour was due to I3. Could someone please explain this question to me, because I can't figure out how the I- was oxidised. Thanks