history -a # append history lines from this session to the history file.
#History file may contain history from other terminals not in this one so:
history -c # clear [in-memory] history list deleting all of the entries.
history -r # read the history file and append the contents to the history list instead.
I've heard that -n can be problematic which is why -c then -r is used.
It looks like it clears your shell's history, and then re-populates it from the .bash_history file?