Take a rectangular piece of graph paper. Place it on one vertex with your favourite reducible Lambda Calculus expression at the top. Apply the first reduction and write the result on the vertex at the other end of the left edge. Apply the next reduction to this new expression and go right this time. Repeat ad finitum. You should have drawn a zig zag path downwards. Reordering the reductions takes you to the same point. This arriving-at-the-same-point-property is called confluence.