This is an economic hypothesis that show a potential relationship between government tax revenue (income) and the rates of general taxation in the economy.
What it simply means is this:-
Punitive tax rates may not generate much additional tax revenue for the government if people respond in ways that result in less taxable income.
So for example if the country raises ist tax rates so high, people may stop working, as it is not worth the effort as the government will effectively take all their income in a form a taxation. Or individuals will resort to tax avoidance strategies to hide their wealth. Thus zero or little tax revenue.
Like wise, if taxes are super low, then people work harder, but then government makes no gain on this hard work and productivity boost, as the taxation rate is so low.
Thus it makes it hard for governments to the right balance on taxation. The curve itself is linked to Arthur Laffer, but Laffer himself has pointed this work to the academtic work from the 14th century Arab Muslim social philosopher Ibn Khaldun