This is the 6552nd most frequent Chinese word.
旋即
"旋即" means "immediately afterward" or "soon after."
Here, '旋即' is used to indicate that the teacher's action of explaining followed directly and immediately after the student's question.
学生说完问题，老师旋即解答了他的疑惑。
The teacher immediately provided an explanation after the student asked the question.
In this sentence, '旋即' describes the immediate cessation of rain and the resumption of their activities without delay.
雨旋即停了，他们继续进行活动。
The rain stopped immediately, and they continued their activities.
'旋即' is employed here to convey that his decision to change plans followed directly and promptly after hearing the news.