쓰다 is the infinitive form of bitter, or "to be bitter", to be exact. Depending on how you use it, it will come in many forms. In this lesson, we are talking about verbal modifiers. In the tips and notes, since it's descriptive, you either add 은/ㄴ (ending in consontant/vowel).
쓴 is the form it takes when it is a modifier. It comes before any subject. 쓴 고양이, 쓴 케이크, etc. Meaning - Salty cat, Salty Cake
써요 is the form it takes when it is in fact, a verb (descriptive). In conjugation (changing the 다 of a verb), we have a rule to remove the 으 and add either 어요, or 아요 in a certain speech level (I forgot the specific name).
In 쓰다, you remove 다 leaving 쓰, but you have to remove 으, which leaves you with ㅆ, and add 어 or 아요. By default if there is no vowel, you use 어요.
Using the examples above 고양이는 씁니다 / 써요. 케이크는 씁니다 / 써요. = A cat is salty. A cake is salty.