It is a short piece of music that precedes the call to worship. Like the preface or introduction in a book prepares you for full understanding of the text before you, so the prelude prepares you for the rest of the service that you are about to partake in. Can you ignore the preface or prelude and still understand and partake in what is happening? Sure. But you will miss something.
Or perhaps think of it in terms of a meal. A feast of a meal. In a five course feast you do not start with the meat and potatoes. Rather you work up to them. You start with something really light that doesn’t demand a whole lot and is almost an afterthought by the time you reach the finale of the feast. It is called an appetizer. The whole point of the appetizer is to awaken your taste buds for what you are about to eat. Could you survive without the appetizer? Yea you probably could… but why would you want to?
In the same way the prelude ought to whet our appetite for ascending to the heavens, to worship God amongst the engels, and to receive his good gifts together.
If we fail to prepare our hearts well and wet our appetites, then the call to worship will be less effective at gaining our attention. Instead of hiking up the mountain entirely you get a ride part way up and then hop out at the start of an intense 2 mile switchback. You have no space to warm up and be ready for it. If we are not prayerfully ready to receive the Lord’s Day service from God, then it may pass us by. Really this preparation ought to begin before you even show up on Sunday. Then as you sit down and hear the prelude begin to be played it will not seem so jarring but rather you will be able to enter into a deeper sense of prayer and readiness to receive the word of God.
The prelude doesn’t have to be complicated. A simple piano piece works perfectly at drawing us in and allowing us to prayerfully seek God and prepare our hearts together. I have even had a trombone lead a prelude before. It can be whatever works for your people to help them prepare their hearts.