The movements of a symphony or concerto are like the chapters in a book. A composer uses them to organize and contrast the themes and ideas in a longer piece of music, and to build suspense or pace the overall expressive contours of the music.
Sometimes in a long piece of music, the musicians and audience all need a moment to digest what they’ve just heard and take a deep breath before moving onto the next movement. Usually, there is a brief pause between movements, but sometimes a composer will instruct the musicians to go directly from one movement to the next without interruption (this is called “attacca”).
Often, a first movement will be lively and upbeat, and set the mood and introduce the different themes that will be heard throughout the piece. Middle movements might be slow and lyrical, or perhaps lighter and more playful in character (called a “scherzo”, which is Italian for “joke”). The final movement is typically another fast and exciting movement that will bring the audience to its feet at the end!
Concertos are usually written in three movements. Symphonies are usually written in four movements, but there are many exceptions to this rule of thumb.