Q:

what does "DependsOn": [] means in cloudformation

I am using a AWS supplied template which uses "DependsOn": [] in a nested stack call.
I know dependson can have a list dependencies but have never come across this.
BucketsStack": {
        "DependsOn": [],
        "Properties": {
            "Parameters": {
                "CreateDemonstration": {
                    "Ref": "CreateDemonstration"
                },

A:

Remove it as it is an empty array so it is useless  . 
Indeed DependsOn affects the order of resources creation. 
Below is from the official documentation. 

With the DependsOn attribute you can specify that the creation of a
  specific resource follows another. When you add a DependsOn attribute
  to a resource, that resource is created only after the creation of the
  resource specified in the DependsOn attribute.

