The dead time is the period at which no part of a H-bridge driver is turned on.
It is used when you have two complementary PWM signals (one on when the other is off) so that neither the high nor the low side of the H-bridge can be conducting at once.
In this waveform it is the grey areas between one signal turning off and the other one turning on.
Since a MOSFET never instantly goes from ON to OFF, or OFF to ON, but instead has a gate charge and discharge time, if you don't include dead time in the waveforms it is possible for one MOSFET to turn on while the other is still in the process of turning off. This gives a low resistance path direct across the power supply, which is a very very Bad Idea™.
For a more realistic view, here's some timing diagrams. As you can see, in the first one, the PWML signal is starting to turn on whilst the PWMH signal is still in the process of turning off. The rise and fall times cause a crossover. In the second one the dead time ensures that PWMH has fully turned off before PWML starts to turn on.
Being able to configure that dead time is important. The more dead time you have the less efficient your converter will be, since during that period it's not actually doing anything at all. But, if the dead time is too short you suffer the shoot through problem and things get messy very rapidly. So tuning it to match the characteristics of the output stage (MOSFET or IGBT, for instance) means you can balance the efficiency to shoot through nicely.