Diagrams like the one above, to represent resonance in pipes are inspired by looking at resonating strings/wires/springs. For this reason, they can be confusing. They should be thought of as sketch graphs of amplitude of oscillation of "layers" of air against position, measured along the axis of the pipe.

In the next diagram, the length of the arrow represents the amplitude of the oscillation of the air at that position in the pipe. Large amplitude at the open end, an anti-node, zero amplitude at the closed end a node of the stationary "wave".

If the full line represents the displacements at a given time then the broken line represents the displacement half a time period later.