This is the 5706th most frequent Japanese word.
たまる
"たまる" means "to accumulate" or "to be saved up."
In this sentence, "たまる" refers to the accumulation of something, in this case, homework.
宿題がたまると大変です。
It is difficult when homework accumulates.
Here, "たまる" is used to express the accumulation or saving up of money over time.
お金がたまるのは嬉しいです。
I am happy when money accumulates.
This usage of "たまる" describes the buildup of a non-physical entity, such as fatigue, emphasizing its gradual increase.