Maz Kanata is a diminutive, Force-sensitive being who runs a castle and a criminal clearing-house on Takodana. The StarWars.com Databank calls her "a diminutive being with vast experience at surviving in the underworld," puts her age at over a thousand years and her height at 1.24 metres, and notes that she "has seen galactic powers rise and fall." Her species is not named in the Databank entry, so this dossier leaves that field blank. She was introduced in Star Wars: Episode VII The Force Awakens (2015), performed by Lupita Nyong'o.
The lightsaber problem
She has Luke Skywalker's first lightsaber in a box in her basement, and how it got there is the single most conspicuous unanswered question in the sequel trilogy. Her own answer — "a good question, for another time" — is either a deliberate refusal or a plot thread that never got picked up, and eleven years on it is still both.
What she is for
She delivers the trilogy's clearest statement of its own theme to Rey: "The belonging you seek is not behind you, it is ahead." Structurally she is the Yoda role relocated from a swamp to a bar, and the relocation matters — she is a fixer with a network, not a hermit with a lesson, and she has been doing business with Han Solo for decades.
Force-sensitive, not Jedi
Like Chirrut Îmwe, she belongs to the small and useful category of characters who know the Force is real without belonging to any order. The Databank is explicit that she is Force-sensitive; nothing in the films suggests training.
Where to read her
The three sequel novelisations and the reference volume Ultimate Star Wars for the castle and its clientele.