What does this do in bash: [@]?

[Bo Berglund]

I am trying to understand the working of a script example I am trying to adapt
but cannot think up a useful google search command to get to a description....

In the script there is this:

URL=('https://tokens.somesite.com' + long stretch of arguments)

Then later:
INPUT=$(curl -qsS "${URL[@]}"  | some processing stuff)

What I do not know is what the [@] in "${URL[@]}" does???

Where can I read bout this?

-- 
Bo Berglund
Developer in Sweden