ceil means if the result is not an integer (..., -2,-1,0,1,2, ...) then round it up to the next integer value (so that you do not need to pay 5.34241513423452 gold each hour, it would be 6 instead.
pow is simply the exponent in computer notation, ie pow($troop, 1.03) = x
1.03.
And using Wolfram Alpha (a lovely tool this one), you can plot it and get:
http://www.wolframalpha.com/input/?i=plot+ceil%28%28pow%28x%2C+1.03%29+*+1.8+-+100%29%2F24%29++x%3D[0%2C100]
Observing it, you see that you start paying with 49 troops (I read it was 36 somewhere else a while ago, maybe the formula has changed? In this post, they have a different formula:http://forum.c-rpg.net/index.php/topic,18074.msg257440/topicseen.html#msg257440)
and then it increases by 1 every 12 troops.
Take into account this formula is just for your own troops, not for fiefs.