What mean "->" in puppetlabs examples on this page
Puppet uses its own DSL to declare the state a system should be in, see "The Puppet Language".
On "ordering", the reference states
Puppet’s language is mostly declarative: Rather than listing a series of steps to carry out, a Puppet manifest describes a desired final state.
The resources in a manifest can be freely ordered — they will not be applied to the system in the order they are written. This is because Puppet assumes most resources aren’t related to each other. If one resource depends on another, you must say so explicitly. (If you want a short section of code to get applied in the order written, you can use chaining arrows.)
Indeed, what you highlighted in the screenshot is a chaining arrow that tells Puppet to do this before that. You mention that you cannot use it anywhere else -- have you got an example that doesn't work for you and where the linked docs don't help?