How to understand Javascript syntax with colon

Question

I am learning Javascript and React. While I read this tutorial: https://suspicious.website/2016/04/29/starting-out-with-react-redux-starter-kit/ there are several syntax that confused me, for example:

export function recieveZen (value: string): Action {  
  ...
}

What are those colons ':' means?

Also the hard part is that: I don't know how to name those syntax to google search for an declaration... so any suggestions dealing with confused javascript syntax for newbie?