This video explains displacement reaction. The reactivity of the halogens decreases as we move down the group. This can be shown by looking at displacement reactions.
When chlorine (as a gas or dissolved in water) is added to sodium bromide solution the chlorine takes the place of the bromine. Because chlorine is more reactive than bromine, it displaces bromine from sodium bromide. The solution turns brown. This brown colour is the displaced bromine. The chlorine has gone to form sodium chloride.
If you look at the equation, you can see that the Cl and Br have swapped places.
chlorine + sodium bromide → sodium chloride + bromine
Cl2(aq) + 2NaBr(aq) → 2NaCl(aq) + Br2(aq).

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