hi! 어렵다 means "to be difficult". verbs ending in ㅂ다 end in 운 instead of 은 when converted to adjectives.
ex: 맵다 "to be spicy" (verb) -> 매운 "spicy" (adjective).
작다 "to be small" (verb) ends in a consonant therefore 은 is used to make it an adjective = 작은 "small"