função x (a) vs função x (... a) a diferença

function x(a)
{console.log(a[0]);
/*for a= ['b','c','d']  a[0] will be b*/
}
function x(...a)
{
console.log(a[0]);
/*for a= ['b','c','d']  a[0] will be ['b','c','d'] to get b you will need a[0][0]*/
/*in second case, the first array has only one term: the a array itself*/
}
Javasper