Taboos are practices that people engage in that are not socially acceptable. During the time and in the place that To Kill A Mockingbird is set, neither incest nor miscegenation would be acceptable practices.
Incest refers to intimate (sexual) relations between near relatives (father:daughter; mother:son, etc.) and during the Depression era, many smaller southern societies and communities even frowned on intimate relations between further distanced relatives (cousin:cousin). In our society incest between near relatives is still illegal and definitely taboo.
Miscegenation refers to intimate relations(sexual intercourse, co-habitation, marriage) between people of different races. In the setting of To Kill A Mockingbird miscegenation is not tolerated and can lead to mob justice and legal action being taken. In society today, miscegenation is not really taboo in larger, metropolitan cities, but some smaller southern societies and communities outside of mainstream America would still consider miscegenation taboo.
The story of "To Kill A Mockingbird" was about Macomb. This was a small southern town during the depression era. During this time in American history the ideas of miscegenation, meaning interracial dating or marriage, was not acceptable behavior. The society in which we lived did not approve of black and white relationships. Black men were hung for sleeping with white women. In many parts of the country today it is still considered taboo, however it does happen more today than it did in the pre civil rights era. The idea of incest was also forbidden. In fact it is still against the law in this country to participate in an intimate relationship with a daughter, son, or sibling. These two types of relationships are not acceptable in Macomb and the fact that Tom Robinson is being charged with rape of a white woman will mean his execution if found guilty.