“Stem” is an English grammar term – it refers to the bit of the word which remains the same when you conjugate. For example, in the conjugation tall -> taller -> tallest, “tall” is the stem.

It’s not exactly the same in Japanese, as the vowel sound at the end of the stem can change between conjugations, but typically when people say “stem” in reference to Japanese verbs, they’re referring to the ます-stem (i.e. the ます-form of the verb with the ます chopped off).