To Stuff In
To Cram In
transitive verb, ichidan verb
Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means stuffed, this word means to stuff in, as in you're doing the stuffing. You know that this is the case because when it comes to the mail (める), you stuff that mail into the garbage as hard as you can (it's all junk).
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You learned that reading with the kanji, so you should have no troubles here.
お弁当にご飯を詰めといてくれない？
Can you put some rice in my bento box for me?