A brain dump (sometimes spelled braindump, or brain-dump) is a complete transfer of accessible knowledge about a particular subject from your brain to some other storage medium, such as paper or your computer's hard drive.
the act or an instance of comprehensively and uncritically expressing and recording one's thoughts and ideas (as on a particular topic)
According to good ol’
Wikipedia, a brain dump is “the transfer of a large quantity of information from one person to another or to a storage and retrieval medium.”
In other words, a brain dump is the act of getting all of your thoughts out of your head so you are able to focus on one idea at a time, and not have to worry about forgetting the other stuff.