Activating the ability with X = 0 will cause all creatures you control to become 0/0 and be put into the graveyard.
Activating the ability for a second time in the same turn will overwrite the first activation. For example, if you activate the ability with X = 4, then activate it with X = 1, each of your creatures will be 1/1.
You apply power/toughness changing effects in a series of sublayers in the following order: (a) effects from characteristic-defining abilities; (b) effects that set power and/or toughness to a specific number or value; (c) effects that modify power and/or toughness but don't set power and/or toughness to a specific number or value; (d) changes from counters; (e) effects that switch a creature's power and toughness. This card's effect is always applied in (b), which means that effects applied in sublayer (c), (d), or (e) will not be overwritten; they will be applied to the new value.
Examples of creature types include Sliver, Goblin, and Soldier. Creature types appear after the dash on the type line of creatures.