what mean sort(function(a,b) in js code example

Example 1: javascript sort array of ints

function sortNumbers(a, b) {
  return a - b;
}
var ages = [12, 95, 43, 15, 10, 32, 65, 78, 8];
ages.sort(sortNumbers); //ages is now sorted low to high
//console.log(ages); [8, 10, 12, 15, 32, 43, 65, 78, 95]

Example 2: sort function

var vehicules= [
   { nom: "Peugeot 208 1,2l PureTech", img:"208.png" , co2:108 },
   { nom: "Peugeot 5008 2.0 BlueHDi 150", img:"5008.png" , co2: 118 },
   { nom: "Golf GTI", img:"golfgti.png" , co2:148 },
   { nom: "Renault Clio 0.9 TCe 90", img:"clio.jpg" , co2:114 },
   { nom: "Audi A4 2.0 TDI 190", img:"a4.png" , co2: 111},
   { nom: "BMW Serie 2 Tourer 220d", img:"serie2tourer.png" , co2:124 },
   { nom: "Audi SQ7", img:"sq7.png" , co2:198 },
   { nom: "Audi TTS", img:"tts.png" , co2:159 },
];
vehicules.sort(function (a, b) {
   return a.co2 - b.co2;
});
vehicules.forEach(function(v) {
   console.log(v.nom+" avec "+v.co2+" g de CO2/km");    
});