What were the contributions of Thomas Malthus to economics ?

Answer 1

As Spyblitz said, Malthus thought population growth would overwhelm food supply. He was wrong because he did not take into account increased productivity in agriculture and he didn't anticipate that birthrates would drop. But that's what he's remembered for and nowadays people use the term "Malthusian" in a derogatory sense to refer to someone or some idea that is stupidly pessimistic.

Answer 2

80 % of the wealth is in 20 % of the people (80-20 Rule)

Food growth is aritmetically, that is 1, 2, 3, 4,...and so on
People growth is geometrically, that is 1, 2, 4, 8, 16 and so on
In a short time people will run out of food