Pangrams ? a task from another website, something in it i have never seen

so Pangrams are strings that have all the alphabet in them, so the website askes to make a piece of code that returns true if a given string is a pangram or false otherwise.
i solved it but my solotion is realy long, then i checked other solutions and found at the top a solution that is no more then 4 lines of code.
the solution have a comparison that i don’t understand.

function isPangram(string){
  string = string.toLowerCase();
  return "abcdefghijklmnopqrstuvwxyz".split("").every(function(x){
    return string.indexOf(x) !== -1;

can somebody explain why he or she compared string.indexOf(x) to !== -1 and how will that check if all alphabets are in the string given to the function?
thank you

I think you are missing some details

when you have a question please include your code and all the relevant information

just pressed enter by mistake in the middle of writing my problem!

what does it mean when indexOf returns -1?

Means no match? The method couldn’t find the index of the given x.
I see something.

Thanx i get it.