using for in for an array in a function

Please explain the code to me for example there is a school with 2 courses. the code let me knows which course is the smallest

enter code here

   const courses = [
  ["Christian", "Annika"],
 ["Julian", "Lisa", "Tobias"] 
        ]


enter code here

     function getSmallestCourse() {


    let smallestCourse = -1
      let smallestIndex = -1

        for (let i in courses) {
      let course = courses[i]

        if (smallestIndex === -1 || course.length < smallestCourse) {
   smallestIndex = i
    smallestCourse = course.length
       }
       }

     return smallestIndex
     }

            console.log("getSmallestCourse()", getSmallestCourse())