plugin generated 751 characters of unexpected output during activation

Your plugin is causing some errors, but not fatal ones, so plugin works, but as the error messages says, it can cause some problems, i.e. the feed that can output the plugin error so not valid xml content, or when somewhere in the code there is a redirect once “headers already sent” it fail.

If you are asking yourself why the plugin works despite that error, it’s because not all errors are fatal (make script die), just as example see following code

if ( ! $number ) {
  $number = 1;
}
echo "My number is " . $number;

If you have a code like this in your plugin it will works, but it causes an error because in first line there is a variable that is not defined anywhere.

This is just an example, but there are a lot of situations like this where code works but causes errors.

When you develop a plugin, activate debug and error log and also I suggest to use some debug tools: some are really useful.