Burst is an Area Effect.

Actions with Burst X effect an area with radius X around the target model.  If the target square model contains a normal model, the shape is as you would expect, for a Burst 1 it would be a 3x3 square area centered around the target.  For large models, unlike Aura X, the burst remains the same size, effectively wasting 3 of the possible squares of effect on the large Model (but the attacker choses which of the 4 squares is the center, provided those squares are valid targets with Line Of Sight).