The word alaap means a dialog or conversation. Alaap is a dialog between the musician and the raga. The purpose of including an alaap in a composition is for the singer to set the scene for the composition itself. In order to tell a story emotionally, the storyteller is required to set the scene for the listener.
The alaap prepares the listener to receive the emotions from the singer. Suspense is also being built up through the alaap for the listener.
In other words, alap is the beginning part of the raga, starts with the key note of the mode it uses, that is “Sa”, both in the instrumental and vocal pieces.
The tempo at the beginning is usually slow in order to show the scale and the key structure of the melody. There is no drum in this section; if it is a piece of vocal music, there are no lyrics with it.