Why does Ralph call a meeting in Chapter 5?
In the novel "Lord of the Flies" by William Golding, the author shows us how Ralph begins to change from childhood ways of thinking into a more adult way of looking at things. The author shows us how law and order begins to break down, and how anarchy can begin to take hold in a society. In Chapter 5 Ralph decides to call a meeting as he senses this is beginning to happen - but does not know his anxieties by this name. All he knows in his childhood heart is that something is not right and he recognises it by its superficial outward signs at first (dirty clothes, torn costumes, shaggy hair, dying fire etc.) He tries to impose or encourage some order or routine to get back to "civilization."
I assume from your tag that you are talking about the meeting that Ralph calls in Chapter 5. I have edited this to show that.
The reason that Ralph calls a meeting in this chapter is because he is unhappy with the way things are going on the island. In specific terms, he is upset about how they missed the possiblity of getting rescued in the previous chapter (because the hunters let the fire go out).
More generally, he is unhappy that the boys do not seem to care about keeping things orderly and organized. He wants them to be more serious about being a bit civilized.