〜ので
because, so (softer, explanatory)
Plain form + ので · Noun / な-adj + なので
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ので gives a reason, like から: 雨が降っているので、出かけません, "it's raining, so I'm not going out".
The difference is tone. から puts your reason forward as your reason. ので presents it as a plain fact that leads naturally to the result, which sounds softer and more polite. That makes ので the usual choice for excuses and requests: 熱があるので、休ませてください.
Nouns and な-adjectives take な before it: 休みなので, 静かなので.
In polite speech you'll also hear ですので and ますので. Among friends, it often shrinks to んで: 雨なんで.
Examples
雨が降っているので、今日は出かけません。
It's raining, so I'm not going out today.
熱があるので、休ませてください。
I have a fever, so please let me take the day off.
明日は休みなので、ゆっくり寝ます。
Tomorrow's my day off, so I'll sleep in.
道が込んでいたので、遅くなりました。
The roads were busy, so I'm late.
この辺は静かなので、よく眠れます。
It's quiet around here, so I sleep well.