Stuck in Basic JavaScript:Stand in Line

I understand what should I do but when I read the instructions that’s given below of it I get confused. What is this instructions means.
nextInLine(, 5) should return a number
nextInLine(, 1) should return 1
nextInLine([2], 1) should return 2
nextInLine([5,6,7,8,9], 1) should return 5
After nextinLine(testArr, 10), testArr[4] should be 10

Hi there, welcome to the forum.

Those are the test cases they use to see if your solution is correct. When you hit “Run the tests,” fCC’s test suite runs behind the scenes to check if your code returns the correct results. If any tests fail, you’ll know that’s where you need to start looking for your mistake.

Hope this helps.