What do it means to see a flat white casket on the floor in a dream?

Although a casket is associated with death, in dreams it is more often a symbol of significant change in some area of life. Something is coming to an end and this ending can be neither stopped or reversed. For example, young people tend to dream of death as graduation approaches. These dreams illustrate the end (death) of childhood and the beginning of their new lives as adults. White caskets traditionally are used for children, so this particular dream probably refers to childhood or something from your childhood that is coming to an end.