It means 'what's this/it about?' 무슨 means what/which/what kind of 일 means work/thing that's going on/thing that's to be done 이야 (i-ya) is the casual form of (i-e-yo), meaning 'is' - the more polite way (i-e-yo) can be used to an official person who stops you to ask you questions, like 'What's this all about/in relation to?' :)

일이야 = 일 + (이)야 일 [noun] The root "verb" is "이다", which is part of 이야.