'.' is equivalent to 'source'. Both are bash builtins, so, check the bash man page to see what 'source' does.
The basic purpose of 'source' is to dump the contents of a shell script on the current shell, instead of spawning a new sub-shell. This is done this way because in bashrc you usually set a number of variables, and setting them into a subshell really makes no sense, since they will go away when the script exits. So, if you want a shell to run on the current shell you 'source' it instead of running it the traditional way.