You gain the following benefits.
You always have the spells on your Spells of the Mark list (if any) prepared.
You have one extra spell slot to cast the spells granted by your Dragonmark feat. The spell slot's level is half your level (round up), to a maximum of level 5. You regain the expended slot when you finish a Short or Long Rest. You can use this spell slot to cast only a spell you have prepared because of your Dragonmark feat or the Dragonmark Preparation benefit of this feat.