what is promise ?

Example of promise

<script>     
//Teacher student ko ak Task (10+20 = ?) dete hain. aur wo student se kahte hain ko jo student Task compleate karega un student men 3 winner hoga aur use teacher prize denge 
    //To bahut se student bolte hain ki sir, Main Task compleate kar dungaa

  //NOTE - yaha pe  do log promise compleate kar rahe hain
       //1. Student = jinka promise hai (10+20) ko compleate kar denge
       //2. Teacher = Jinka Promise hai ki 3 Winner ko Prize denaa

  // Promise Create syntax
    //new Promise((resolve, reject)=>{})
  // .then((argument_for_resolve)=>{}).catch((argument_for_reject)=>{})

    let Task = 10 +20;
    let studentTask = new Promise((resolve, reject)=>{
      setTimeout(()=>{
        if(Task == 30){
          let winner_student = ["Ram Kumar", "Suresh Sinha", "Mayank Sharma"]
          resolve(winner_student)
        }
        else{

        }
      },3000)

    })

    let teacherpromise = (winner)=>{
      return new Promise((resolve, reject)=>{
        setTimeout(()=>{
          let prize = {
            first : "Laptop",
            second : "Mobile",
            third : "Camera"
          }
          resolve(`
            ${winner[0]} is First Winner and his Prize is ${prize.first}
            ${winner[1]} is Second Winner and his Prize is ${prize.second}
            ${winner[2]} is Third Winner and his Prize is ${prize.third}`)
        },3000) 
      })
    }

    studentTask.then((x)=>{
      
      console.log(x)
      teacherpromise(x).then((prz)=>{
        console.log(prz)
      })


    }).catch((y)=>{
      console.log(y)
    })
</script>