さ is a 感動詞（かんどうし） (interjection) in Japanese that has three primary uses. These are as follows:
誘（さそ）い、促（うなが）し - To invite, incite, or draw attention to something.
驚（おどろ）き、当惑（とうわく）し - To express surprise, confusion, or bewilderment.
話（はなし）の流（なが）れ
を抑（おさ）える - To control or keep in check the flow of a conversation.
In this lesson, we will be focusing on the first meaning (to invite or draw attention). In English, this is usually translated as ‘there we go’, ‘ok then’, or ‘well you see’. さ (or sometimes さあ) will come at the beginning of a sentence, before the point that the speaker would like to highlight is explained.
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さーさー、遠慮（えんりょ）をせずにどんどん食（た）べてください。
There we go! Please eat a lot without holding back.
さあ、そろそろ行（い）きますか。
Ok then, should we go soon?
This
さ and the
さ used for expressing confusion or bewilderment are often used interchangeably. When drawing attention, it comes across like ‘well you see’, but when expressing confusion, it is closer to ‘well let me see’. The latter has a slight nuance of ‘let me think about my answer first’.
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あのさ、昨日（きのう）は暑（あつ）さに負（ま）けて倒（たお）れちゃった。
Well, you see… I lost to the heat yesterday and passed out. (Yesterday, I passed out due to the heat)
さあ、それはどうでしょう？私（わたし）に聞（き）いてもわからないよ。
Well let me see… I don't know about that. Even if you ask me I wouldn't have a clue.