Different outputs for the list in Google Earth Engine

Question

I defined the following list on GEE,

var List = ee.List.sequence(ee.Number(1), ee.Number(100), 2);
print('My list is', List);
print('My list is' + List);

For the first print(), output is,

My list is
List (50 elements)

And for the second one,

My list isee.List({
  "type": "Invocation",
  "arguments": {
    "start": 1,
    "end": 100,
    "step": 2
  },
  "functionName": "List.sequence"
})

I'm wondering why this happens and what + object in the argument of print function does this? Unfortunately I couldn't find a document about this.