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It's a form of hair straightener. People of African descent tend to have "kinkier" hair1. In the 1940s, it was common for some African Americans to emulate the straighter style as seen on European or "white" people.
In order to do this, some African Americans used congolene - a gel-like substance that contained lye - to "cook" their hair until it was limp. This style was known as the "conk2". Congolene could be made at home from potatoes, eggs and lye3 and was applied to the hair like shampoo. Since lye is a very corrosive substance, it was washed off very quickly after being applied and the hairdresser would wear protective gloves. The resulting hairstyle would be limp, straight hair which could then be styled further.
Congolene and Conk hairstyling started to die out in the 1960s/70s with the advent of "natural" Afro hairstyles being embraced by African Americans, as well as the mainstream cultural acceptance of these styles. Hair straightening products for less-extreme "straight" styles such as the Jheri curl4 were also a lot more safer.
References:
1: http://en.wikipedia.org/wiki/Natural_hair
2: http://en.wikipedia.org/wiki/Conk
3: http://www.scribd.com/doc/29978/My-First-Conk
4: http://everything2.com/index.pl?node_id=826118
