This is the 23rd most frequent Japanese word.
ので
"ので" is a conjunction meaning "because" or "since," used to indicate a reason or cause.
In this sentence, 'ので' is used as a conjunction to indicate a causal relationship between the fact that it will rain and the decision to bring an umbrella.
今日は雨が降るので、傘を持って行きます。
It will rain today, so I will bring an umbrella.
Here, 'ので' expresses the reason for the advice given, linking his busyness to the suggestion to avoid speaking to him.
彼が忙しいので、今は話しかけない方が良いです。
Since he is busy, it's better not to talk to him now.
In this context, 'ので' connects the lack of time with the proposed action of hurrying, showing causation.