The reactivity of the halogens decreases as you go down group 7. Chlorine is more reactive than bromine, and bromine is more reactive than iodine. You can show this with displacement reactions: a more reactive halogen takes the place of a less reactive halogen in a solution of its halide.
If chlorine solution is added to potassium bromide solution, chlorine displaces bromine and the mixture turns orange: Cl2 + 2KBr → 2KCl + Br2. Chlorine also displaces iodine from potassium iodide solution, giving a brown colour. Bromine displaces iodine from potassium iodide solution, but bromine does not react with potassium chloride solution, and iodine does not react with potassium chloride or potassium bromide solution. There is no reaction when the added halogen is less reactive than the halogen already in the halide.
In the practical, you add a few drops of each halogen solution to each halide solution and note any colour change. Astatine lies below iodine in group 7, so it is the least reactive of these halogens. We predict that astatine would not displace chlorine, bromine or iodine from their halides, but each of those halogens would displace astatine from a solution of an astatide.