The meaning of Burst Mode
Normally with a buck regulator it continually switches a transistor on and off at a certain duty cycle to feed energy to the load. When the load draws high current the energy transferred is high and this results in a high duty cycle i.e. the transistor is on more than it is off.

When the load is very light and the input voltage is at the high end of its working range, you find that a very small duty cycle (less than 1%) is difficult to produce and so some regulators switch off the main process of continuous duty cycle and go "idle" for a while. During this period of idle, the output voltage drops to a certain point and this triggers to process to restart until the voltage reaches a higher level. 

Then the regulator goes idle again for a while. This idle period is very much dependent on load current - if there is very little load current the idle period can be several milliseconds or more.

This is burst mode.