Some cards have abilities that you can activate while in your hand. You can use such abilities during your Main Phase without worrying about link symbols, because such abilities do not require you to actually play the card. Also, in order to use such abilities, you must have them in your hand. You cannot use these abilities after you’ve played the card in your field.
In this basic set the only card with such an ability is the Curse Witch, but the Northern Enchantress expansion will introduce more.