This section is about a number of cards that are called staples, or cards that should be in most decks. That doesn't mean they should be put in every deck you have, of course, but all of them are extremely useful.
1) Dark Hole: This spell card destroys every monster on the field, including your own.
Why it's a staple: This card can potentially change a game, clearing your opponent's field if you're currently getting dominated.
2) Heavy Storm: This spell is similar to dark hole, but destroys all spells/traps instead.
Why it's a staple: Again, it's similar to Dark Hole in that it can destroy your opponents backrow when you don't have one, ruining any lead they originally had
3)Solemn Warning: This counter trap cards has a cost of 2000 life points, and negates the summon of a monster, or a spell/trap that would summon a monster(this includes cards that do not have to special summon a monster, such as starlight road).
Why it's a staple: Life points are abundant, especially at the start, and negating your opponent's moves is always a great thing. Further, since this is a counter trap, only other counter trap cards can be chained to it.
4)Solemn Judgement: This counter trap card requires half of your life points, and negates a monster summon, spell, or trap.
Why it's a staple: Similar to warning, is it good to negate your opponent's plays, especially since you have plenty of life points.
5)Monster Reborn: This spell card allows you to special summon any monster from either players graveyard, providing the monster allows you to summon it.
Why it's a staple: This is a potential game-changer, letting you revive a monster essentially for free. Whether you use it to summon one with high attack, or an amazing effect, or what have you, this card can be extremely useful in many ways.
Those are 5 cards that are put in the majority of decks. All 5 of them are limited, meaning you can only have 1 of each in you deck, showing just how powerful these cards can be.