The subordinate clause is a part of a sentence that depends of the main part, and works whithin it as an adjective, an adverb or a noun.
In the sentence:Allison and Andy want to travel to India someday once they saved enough money for the trip.
The nucleus of the sentence will be allison and andy want to travel to india someday and the advervial subordinate clause within the sentence will be "once they saved enough money for the trip".
This way we see how the second part of the sentence by itself has not too much sence but once the two parts make one sentence we see the full sense and meaninf in the sentence