You didn't mention which part you need help with so let's break the whole card down.
2 Tuners + 1 non-Tuner DARK Dragon-Type Synchro Monster
This is the recipe, pretty self-explanatory.
When this card is Synchro Summoned: You can activate this effect; for the rest of this turn, your opponent cannot activate cards, also cards your opponent controls cannot activate their effects. Your opponent cannot activate cards or effects in response to this effect's activation.
This is its first effect. It's an optional Trigger effect that activates when Calamity is Synchro Summoned. Note that it's a "When" optional Trigger effect, so it will miss timing if the last thing that happens isn't its activation condition, in this case, the Synchro Summon of Calamity.
What the effect does is that after it has successfully resolved
, for the rest of that turn, the opponent cannot activate any cards (like activating a Spell Card from their hand or an already Set Trap Card on their field), and also, cards they control (control = on their field) cannot activate their effects (like monsters on the field or Continuous Traps that already face-up on their field). Note that this doesn't stop or negate Continuous Effects from applying, such as a card like "Umi" increasing/decreasing the ATK/DEF of monsters depending on their Type.
The last part is a condition that states that when you activate this effect, the opponent cannot chain anything to it specifically (whether it's activating a Card or activating an effect of a card).
If this card destroys a monster by battle: Inflict damage to your opponent equal to that monster's original ATK.
This is its second effect. It's a mandatory Trigger effect that activates if Calamity destroys a monster by battle. The effect itself is pretty self-explanatory, it does effect damage to the opponent equal to the destroyed monster's original ATK.
If this card in its owner's possession is destroyed by an opponent's card (by battle or card effect): You can target 1 Level 8 or lower DARK Dragon-Type Synchro Monster in your Graveyard; Special Summon it.
This is its third and final effect. Again, it's an optional Trigger effect that activates when Calamity in your (it's owner's) possession is destroyed by an opponent's card (either by battle or by card effect). Note that it's an "If" optional Trigger effect, so unlike the first effect, this one cannot miss timing. "Possession" is a bit different than "control" in that it's not only cards on the field. It also includes cards in that player's hand, Deck, Extra Deck, Graveyard, their banished cards, cards attached as Xyz Material to an Xyz Monster that player controls and cards that are in the middle of being Summoned to that player's field. So, this effect can be activated even if your Calamity is destroyed by a card effect while in your Extra Deck.
The effect is again, pretty simple, it targets a Level 8 or lower DARK Dragon-Type Synchro Monster in your Graveyard and Special Summons it (note that Synchro Monsters need to be properly Synchro Summoned first before you can revive it from the Graveyard or banished).
Hope this breakdown helped.