According to Malthus, the supply of food increases arithmetically, while the population increases geometrically. This means that the population grows faster than the food supply, which can lead to poverty and starvation.
An arithmetic increase is a linear increase, where the amount added to the total each time is constant. For example, if the population of a country is 100 million and it increases by 1% each year, the population will be 101 million the next year, 102 million the year after that, and so on.
A geometric increase is an exponential increase, where the amount added to the total each time is proportional to the total. For example, if the population of a country is 100 million and it increases by 2% each year, the population will be 102 million the next year, 104.04 million the year after that, and so on.
As you can see, the geometric increase is much faster than the arithmetic increase. This is why Malthus believed that the population would eventually outstrip the food supply, leading to poverty and starvation.