what's the difference between executing a program by simply typing it's name, vs. executing a program by typing a . (dot) ?

source, . (dot command)

This command, when invoked from the command-line, executes a script. Within a script, a source file-name loads the file file-name. Sourcing a file (dot-command) imports code into the script, appending to the script (same effect as the #include directive in a C program). The net result is the same as if the "sourced" lines of code were physically present in the body of the script. This is useful in situations when multiple scripts use a common data file or function library.

Running "runme" will create a new process which will go on its merry little way and not affect your shell.

Running ". runme" will allow the script "runme" to change your environment variables, change directories, and all sorts of other things that you might want it to do for you. It can only do this because it's being interpreted by the shell process that's already running for you. As a consequence, if you're running bash as your login shell, you can only use the "." notation with a bash script, not (for example) a binary on C shell script.