to sing: to lift the voice in song, most often in praise to God or in celebration
At its core this is the ordinary act of making music with the voice. In the Bible that act is overwhelmingly tied to worship and to joy: people sing to God, sing of his deeds, and sing at moments of victory, festival, and gladness. The very first time the word appears, Israel sings at the Red Sea (Exodus 15:1), and from there it runs all through the Psalms as the natural response of a grateful or hopeful heart.
The word stretches in two directions that still share this one center. Most of its uses describe the act itself: a person or a crowd raising a song. But its "singing" form also hardened into a title for the people whose job was to sing, especially the trained temple singers who served in worship. They are still simply "the singing ones," yet the word names a settled role, not just a one-time act.
Singing here is rarely idle entertainment. Even where the singers are at a royal court or a banquet, the weight of the word falls on song as celebration and praise. To sing, in this vocabulary, is to give voice to gladness, gratitude, or honor.