there is no exact meaning of ㅁ. It's just one of the Korean alphabet. Same with English there is no meaning of "m" right? 있음 is from 있다 which is dictonary form. 있다 means "there is (to be exsistance)" or "I have"

고마워요, 현경씨.. ^^

If the stem ends in a (double) consonant, like 있다(to have / to exist / there is...), 먹다(to eat), you can put an 음 after the stem, to create a noun. So you'll get 있음, 먹음. I don't think you need to know more than that right now since it's not common in everyday Korean. :)

Thanks for answering^^ But mostly I was confused about 있음... Haha.. >.<

way*