I've gotten a few as quest prizes and have no idea what they really do/how they work? I know the description should be self explanatory but I need help lol /: Is there a best time to use them (like weekend bonus quests) or should I sell them?

When you click the "Free CSC" option on the item it activates a buff which gives you +5 CSC when you complete a quest for up to 5 quests. You don't need to save it for weekend quests or anything special, just use it right before you do 5 quests. Essentially, each card is worth 25 CSC.

Oh, awesome. thanks for your help!

You're welcome :)