Korean verbs and adjectives consist of two parts; "어간" is the former part which always remains the same. The latter part is called "어미" which changes into various forms in different contexts, usually according to the tense, respect form, etc.
The basic form "eat" is "먹다". When it is used in a sentence, it chanes into "먹고", "먹어서", "먹는", "먹었다", "먹자" and so on. Regardless of how it changes, "먹~" remains the same. So, the stem part "먹~" is the "어간", and the rest ending part is called "어미".