A tribe is an association of people with common ways of thinking and doing things and a sense of shared identity. Typically tribes are organised as collections of smaller associations called clans. Clans usually have recognised leaders who are seem as responsible for the welfare of the members of the clan. Tribes don't normally have strong leaders themselves but tribal affairs are governed by a council of clan leaders or other responsible people.
Clans are like large extended families in which members look out for each other. For this to work people need to know the other members of the clan at least by sight. Thus there is a natural limit to the size of a clan which some commentators put at around 500 people.
When people organise as clan and tribe they become part of project larger than their individual self or nuclear family. Whereas a nuclear family is too small to organise all the means of life - such as food growing & harvesting, food preparation, clothes making, shelter building, care and education of children, care of the sick, craft activities, cultural activities and defence - a clan is potentially large enough. Thus if left alone and given sufficient resources a clan can lead an autonomous life. Because the leaders of the clan know the adult members of the clan personally they are more likely to pay attention to their needs than they would if this was not the case. Also because all of life is happening at a small scale it is easier for clan members to make sense of the whole picture and keep a check on things. Thus there is a fair chance that the clan will develop in ways that suit its members and provide them with a good life.