A stem is the part of the word that doesn't change even when the verb/adjective is morphologically changed. However for the Korean language the stem is the part before the 다 part in the base form of the verb/adjective since irregular verbs can modify the base form (i.e. ㅂ and 르 irregular verbs).
Most of the verb/adjective conjugations in Korean are done so by simply adding to the stem.
Irregular verbs can modify the stem