Reparations are money that one entity (usually a government) has to pay to another entity (a government or perhaps a group of people) because it has done something wrong to them. A government can be forced to pay reparations or it can pay them voluntarily because it feels that it has to do something to make up for the wrong that it has done.
An example of forced reparations is the reparations that Germany was forced to pay to France and England after World War I. Germany was blamed for starting the war and was therefore forced to make payments to make up for the damage caused by the war.
An example of voluntary reparations came when the American government voted to pay reparations to Americans of Japanese descent who had been interned in World War II. The Congress decided that the US had done a great wrong to these people. Therefore, it decided to pay reparations to them without being forced to do so. Some activists would like the government to pay reparations for having allowed slavery to continue for so long, but that is very unlikely to happen.
Reparations are a payment, whether in currency or goods and services, that are made by one party to another in response to an injury inflicted by the former on the latter. A classic example involves German reparations to Israel for the former's infliction of crimes against European Jews during the 1930s and, especially, the period of World War II. Germany has aided the State of Israel since its founding in 1948 with financial assistance but also by selling to Israel military techology to help the Jewish state defend itself.
In the United States, there has been debate about whether the federal government should pay reparations to the African-American population to compensate the latter for slavery and the absence of civil rights until the Civil Rights Act was passed in 1964.