Special Traits

Special traits (which appear after a monster's challenge rating but before any actions or reactions) are characteristics that are likely to be relevant in a combat encounter and that require some explanation.
Innate Spellcasting

A monster with the innate ability to cast spells has the Innate Spellcasting special trait. Unless noted otherwise, an innate spell of 1st level or higher is always cast at its lowest possible level and can't be cast at a higher level. If a monster has a cantrip where its level matters and no level is given, use the monster's challenge rating.
An innate spell can have special rules or restrictions. For example, a drow mage can innately cast the Levitate spell, but the spell has a "self only" restriction, which means that the spell affects only the drow mage.
A monster's innate spells can't be swapped out with other spells. If a monster's innate spells don't require attack rolls, no attack bonus is given for them.

Spellcasting

A monster with the Spellcasting special trait has a spellcaster level and spell slots, which it uses to cast its spells of 1st level and higher (as explained in the Player's Handbook). The spellcaster level is also used for any cantrips included in the feature.
The monster has a list of spells known or prepared from a specific class. The list might also include spells from a feature in that class, such as the Divine Domain feature of the cleric or the Druid Circle feature of the druid. The monster is considered a member of that class when attuning to or using a magic item that requires membership in the class or access to its spell list.
A monster can cast a spell from its list at a higher level if it has the spell slot to do so. For example, a drow mage with the 3rd-level Lightning Bolt spell can cast it as a 5th-level spell by using one of its 5th-level spell slots.
You can change the spells that a monster knows or has prepared, replacing any spell on its spell list with a spell of the same level and from the same class list. If you do so, you might cause the monster to be a greater or lesser threat than suggested by its challenge rating.

Psionics

A monster that casts spells using only the power of its mind has the psionics tag added to its Spellcasting or Innate Spellcasting special trait. This tag carries no special rules of its own, but other parts of the game might refer to it. A monster that has this tag typically doesn't require any components to cast its spells.