Ralph calls the twilight assembly in response to a perceived breakdown of order amongst the boys; particularly the disastrous neglect of the fire which saw a possible chance of rescue slip by. He is nervous but determined to stamp his authority as he senses that life on the island is not how they had originally pictured it to be,
The time had come for the assembly and as he walked into the concealing splendours of the sunlight he went carefully over the points of his speech. There must be no mistake about this assembly, no chasing imaginery..... (p.95)
At the meeting Ralph covers points ranging from the organization of drinking water to addressing why it is that so many of their number are feeling frightened. The latter problem is an issue that sees the assembly slide into chaos with much discussion of the possibility of 'a beast' lurking on the island. Try as he might, Ralph is unable to hem in the scope of discussion about a beast and ultimately the meeting ends in rancour with Jack showing no regard for Ralph's authority. Ralph's worst fears of the meeting being a failure are realized and he is left feeling completely dejected.
Perhaps the three most important problems he raises at the meeting are keeping the signal fire burning (which the group largely agrees to), doing any cooking at the top of the mountain so as to maintain the signal fire (which causes much dissent) and addressing how to deal with 'the fear' (which ultimately sees the meeting completely break down). It is clear by the end of the meeting that the boys will have little regard for the rules Ralph has pushed to solve the problems, and perhaps the signal fire is the only area in which he might still stamp some authority.
Perhaps, you mean the assembly Ralph conducts during chapter 5. He raises various points in this meeting. First of all, he raises the issue of team effort. The only two people who truly made the shelters were Simon and Ralph, they have to work together to get things done in order to survive. Also, he raises the issue of keeping the fire tended. In the previous chapter, Jack let the fire go out in the pig hunt, which was important because they had a slight chance of rescue. Also, to designate a area to pee and poo. The boys, especially the litteuns are making the camp quite dirty, so it is important they have an area just for that. These are only 3 issues that Ralph raises at the assembly.