Making sense of TI power-designer sims
The waveform that you're seeing labeled transient response shows how the output voltage fluctuates due to a changing demand in current downstream from the converter. This can tell you a few things such as: stability, overshoot / undershoot, response time, and more. 

When the red waveform (load current) goes from 3A towards 0A, the energy that was stored in the inductor L1 doesn't change instantaneously, and therefore must be absorbed into the output capacitor Cout. This results in a voltage increase in the voltage output. The opposite change in load current causes the voltage at the output to drop since the power converter cannot respond as quickly as the load is being demanded. The energy is temporarily supplied by Cout and causes a dip.

Usually you want to know how the output responds under rapid load changes of different frequencies so you can determine if your output voltage is stable and within the max/min voltage of the downstream parts that power is supplied to.

This is a very brief explanation, but hopefully gives you some insight.