Recitative (Italian: “recitativo”) is music which is telling a story quite quickly, as if it were being spoken, "talky". The word means: “to recite” i.e. to tell a story.
Recitative is used in opera, oratorio and cantatas. When opera was invented around 1600 the composers needed to tell the story in music. In recitative the story is sung quickly, with maybe just a harpsichord playing a few chords. After a while, the situation in the story has changed, and the singer can sing an aria which is more interesting musically.
When recitative is just accompanied by a keyboard instrument it is called “recitativo secco” (dry recitative). Sometimes the orchestra joins this. This is called “recitativo accompagnato” (accompanied recitative). There are no bar lines in recitative because there is no regular beat.
Recitative is simple musically, it can sometimes describe the words being sung in quite interesting or amusing ways. Sometimes this might be improvised by the harpsichordist.
In the 19th century the difference between aria and recitative gradually disappeared. Wagner wrote operas where everything had musical interest and the various sections flowed into one another.