Moses had a credibility problem with his fellow Israelites, because he had murdered an Egyptian and fled. So the Lord God promised Moses that he would make it so that the elders listened to his voice. The Lord would show Moses that he was on his side, through the support that the more experienced, wise, and understanding representatives of the people would give to him (see Exodus 18:1–27 for development of this idea).
18 And they will listen to your voice, and you and the elders of Israel shall go to the king of Egypt and say to him, ‘The LORD, the God of the Hebrews, has met with us; and now, please let us go a three days’ journey into the wilderness, that we may sacrifice to the LORD our God.’