안녕하세요! I just started learning Korean, too :) A batchim is a term used for "final consonant". For example, 한글 (hangeul) ends with ㄹ which is a consonant, so this word has a batchim; 한국어 (hangugo) ends with ㅓ which is a vowel, so it doesn't have a batchim. You will also see "double batchim" which are batchims like ㅅㅅ, ㅂㅂ etc. 안녕 :)

So far as I understand, Patchim/batchim is the third letter in a syllable. The ㅁ in 감.