Be good to people and humble as you rise in life because fortune may change and you will need those people's support. Another way to put it is, do good to people on the way up because you may meet them on your way down.
The proverb emanates from a folk tale or fable, which are an integral part of many African cultures including Ndebele. In this tale, a story is told of man called Manyosi who was favoured by the rulers of the time. Due to this favour Manyosi had plenty to eat so much that he developed a big belly. Later on he fell out of favour with his superiors and lost all the supplies which contributed to his big belly. The people started mocking him saying his stomach will grow smaller coz of hunger and starvation. They did not want to support him either because he had been very proud and distant from the people when he was still favoured. In life, some people will always boast of their fortunes and act or behave as if that fortune is due to their wisdom or cleverness. However there are times when fortune will run out and people are left with no one willing to help them due to the way they treated others during their time of abundance.
The proverb is therefore aimed at consceintizing or reminding people on the importance of humility and treating others well during times of abundance or fortune - mindful that one may run out of that fortune or abundance.