When a white person claims to have 'discovered' something that already existed. Like Columbus did with the Americas. It usually entails taking credit for something that may be part of someone's culture and might already have been there, such as black fashion, African American slang, or twerking and dreadlocks.
Christopher Columbus is acclaimed for having 'discovered' the new world. Him, an European white guy backed by the Spanish crown just waltzed in and took over the land and the people who had been there for thousands of years with their culture and form of life. Him and his men brought illness and suffering and Christianity to the 'savages'.
A similar trend is seen when white, middle-class people move into neighborhoods that are predominantly non-white and of a lower socio-economic level and hype up the prices, kicking out the inhabitants who've lived there for generations. Gentrification is a form of Columbusing.
Whenever a white dude wear dreads, like most white people who don't know the cultural and religious implications of doing so, they are guilty of Columbusing.