What is $ in $.ajax used for?

Question

I am getting an error which says

$ not defined

How do I define it?

function temp(){
  
$.ajax({
        url: "http://localhost:8000/sendmessagelol",
        type: "POST",
        data: {test:"test data lolololol"},
        headers: {
            token: token
        },
        success: function(response){
          console.log('response: ', response)
          callback(response);
        },
        error: function(response){
          console.log('response: ', response)
          callback(response);
        }
      });
    console.log(abcd);
}

document.getElementById('clickhere').addEventListener('click', temp);