Check my spelling as I type option offer you check spelling of words are typed in case of error (different from the language system used).
After the Check my spelling as I type process, each error type (different from the language system), then the word will contain jagged red line. See the picture below as example. I wrote a Facebook status in the Indonesian language and the jagged red line appears. I really do not like it and find ways to eliminate it.
I use Palemoon internet browser based on Mozilla Firefox. That option can be found in Advanced, then looking for the first tab menu (General). In Browsing section, you will found option Check my spelling as I type. You need to remove the check mark on that option. For more clearly, please find the the images below.