The only options you usually need for comm are -1, -2, and -3. It takes two arguments, call them 1 and 2. Its job is to include lines from each and both files in output (by default, you get 3 columns of output), so we want to filter by telling it to exclude what we don't want. My mnemonic is "-" means "not", so -1 means "not only in file 1", -2 means "not only in file 2", and -3 means "not in both". So now, when I want comm output that shows the lines unique to my 2nd file, it's:

comm "not only in 1" and "not in both"

so:

comm -1 -3 file1 file2