Here's an example:
curl -s acme:acmesecret@localhost:9999/uaa/oauth/token
It looks like it is used to specify that the URL contains the acme:acmesecret (authentication credentials) part?
Reference
As Charles pointed out, these types of questions are very easy to answer with explainshell. So if you ended up here you will probably have other similar questions and explainshell is the way to go.