This is the 3809th most frequent Japanese word.
またがる
"またがる" means "to straddle" or "to span."
In this sentence, 'またがる' functions to describe the act of sitting astride or mounting, particularly onto a horse.
馬にまたがるのが難しいです。
It's difficult to mount a horse.
Here, 'またがる' is used to indicate the idea of spanning or stretching over something, in this case, a bridge over a river.
この橋は川にまたがっている。
This bridge spans over the river.
In this context, 'またがる' conveys the meaning of extending over a period, referring to time or duration.