You can easily make standing waves yourself by taking a rope, fixing it on one end, and shaking the other end up and down. If you play around you'll find that at a specific frequency of vibration, one big peak forms in the center of the rope. That's because as one wave peak travels down the rope and reflects off the end, the next peak meets it wave at the same spot resulting in a "double wave". This adding of wave peaks is called constructive interference. If you double your original "fundamental" frequency you'll get two big peaks with a flat spot in the center. The big peaks (antinodes) occur where the travelling waves are in phase and constructively interfering, while the flat spot (a node) occurs where the peaks are exactly out of phase. The more standing wave peaks in the rope the higher the "harmonic."