Definition:
Data Flow Diagram (DFD) is a graphical representation of the “flow” of data through a system creating an overview without going in to detail which can be elaborated. DFD diagrams helps in collecting and understanding requirements as well as designing systems.
DFD is visualization technique which enables communication and collaboration of key information. Shows what kind of information will be input and output from the system, how data flow through the system and where data will be stored. DFD helps users to visualize how the system will operate, what the system will accomplish and how the system will be implemented.
This modeling helps in developing any system, visualizing major steps and data involved in system processes in a top-down approach to system design also known as Bubble Chart.
Further Reading:
https://en.wikipedia.org/wiki/Data_flow_diagram
http://tracks.roojoom.com/r/14373
https://www.visual-paradigm.com/guide/data-flow-diagram/what-is-data-flow-diagram/