javascript console -infinity, what does it mean?

The first argument of Function#apply is thisArg and you are just passing the thisArg as the array that means it's calling Math#max without any argument.

As per MDN docs :

If no arguments are given, the result is -Infinity.

In order to fix your problem set Math or null as thisArg.

let max= Math.max.apply(Math, numeros );

let numeros= [1,5,10,20,100,234];
    let max= Math.max.apply(Math, numeros );
    
    console.log( max );

As @FelixKling suggested, ES6 onwards you can use spread syntax to supply arguments.

Math.max(...numeros)

let numeros = [1, 5, 10, 20, 100, 234];
let max = Math.max(...numeros);

console.log(max);