`reduce` makes that function much more complex-seeming than it actually is. (`reduce` is hugely overused, and almost always the wrong tool.) Here's the same function without the unnecessary `reduce`, with explanation:

function groupBy(objectArray, property) {
  // The object we'll return with properties for the groups
  let result = {}
  // Loop through the array
  for (const obj of objectArray) {
    // Get the key value
    let key = obj[property]
    // If the result doesn't have an entry for that yet, create one
    if (!result[key]) {
      result[key] = []
    }
    // Add this entry to that entry
    result[key].push(obj)
  }
  // Return the grouped result
  return result;
}

The `reduce` version just passes `result` around (as `acc`) by receiving it as the first argument to the callback (because that's how `reduce` works) and then returning it from the callback.