I believe you are asking about displacement nodes and antinodes. Right at the closed end, a sliver of air dx cannot oscillate because it has nowhere to go which causes the pressure to rise and drop above as far as it will ever go above ambient pressure. You have a displacement node and a pressure antinode.
At the open end, the opposite happens; the sliver of air is free to oscillate at maximum displacement while the pressure is fixed at its ambient (atmospheric) value. You have a displacement antinode and a pressure node.