Cannot understand this notation using javaScript reduce method

Question

having this array:

const users = [
  { firstName: "lala", lastName: "diaz", age: 23 },
  { firstName: "lela", lastName: "gil", age: 54 },
  { firstName: "lila", lastName: "mendez", age: 43 },
  { firstName: "lula", lastName: "lopez", age: 23 }
]

I have to find the amount of people having different ages. Output: acc = {23: 2, 54: 1, 43: 1}

The following algorithm works perfectly:

const output = users.reduce((acc, curr) => {
  if (acc[curr.age]) ++acc[curr.age]
  else acc[curr.age] = 1
  return acc
}, {})

I understand that the initial value, acc is an empty object, but I do not understand the acc[curr.age] notation. I mean: Am I creating an temporal array called arr, and each element of that array arr is stored as key value pairs in each iteration of the reducer function? is that the case? how is it done?