The cardinal numbers are one, two, three, unu, du, tri. The ordinal numbers are first, second, third, unua, dua, tria.
Esperanto happens to use ordinals to talk about time. "It's two o'clock" = "It's the second hour" = "Estas la dua [horo]". Since kiom is a question asking for a (cardinal) number, kioma is used to ask for an ordinal.
It'd be as if in english we made a distinction between "which" and "whichth", which I love saying just because I love trying to pronounce "whichth".
You might also use kioma to ask which of a number of things in a sequence has some property or whatever.
- Kioma filmo "Reen al la Estonteco" estas via plej ŝatata? - La dua. (which "back to the future" movie is your favorite? the second)
Very probably if you said "Kiu filmo" instead of "Kioma filmo" it'd still make perfect sense, but I think it's cool that kioma nails down the meaning of the question so nicely.
Because "horo" is a noun, it takes here the adjective "kioma" to qualify it in this particular question.
Check this grammar: http://www.gutenberg.org/files/7787/7787-h/7787-h.htm (use Ctrl+F to search for item 185., "Time of day").