MDC or Mega-Damage Capacity is a higher level of damage that can be taken and given.
Not all settings will have MDC in it. Settings that do: Rifts and Phaseworld.
1 MDC is equal to 10 SDC.
Weapons or attacks that do less than 10 SDC cannot damage MDC at all in general.
Things that might have MDC are Tanks, Power Armour, Cyborgs, Dragons, Supernatural Beings like Demons. Weapons that may do MDC are things like Particle Beams, Plasma weapons, Missiles, Dragon Claws, etc.
See individual settings for specific items.