The word d'oraita, is often used colloquially to mean "very serious". You might say, half-jokingly, that parking on a double-yellow line or sitting in your father's chair are issurim m'doraita. (In fact the latter actually is.)
The Aramaic oraita means Torah or instruction, and derives from the Hebrew or, which means light. Therefore a law originating d'oraita means a law that is based directly on the Torah.
Laws of rabbinic origin are called d'rabbanan, from our rabbis. These are also binding and the rabbis' authority to make them is rooted in the Torah (see Deuteronomy 19:17).
One application of the difference is when you are in doubt as to whether you have fulfilled a certain mitzvah. If it is a rabbinic commandment, you follow the principle safek d'rabbanana l'kula and are lenient. Therefore, if you are uncertain whether you have made a blessing before eating, you do not say a berachah just to be on the safe side, for that risks making an unnecessary blessing which would involve invoking God's name in vain - a d'oraita prohibition.
The converse is safek d'oraita lehumra: a d'oraita doubt requires erring on the side of stringency. If you are uncertain whether you have recited the Shema - a d'oraita command - you must repeat the prayer.