Given I had no knowledge of rebar3 before clicking the link, it took me a while to discover what it actually is.

There is no descriptive text on the landing page aside from "Build Better Erlang" which i guess narrows this down to some sort of erlang developer tool. The Getting Started page has no mention of what we are installing actually does aside from a snippet in their console output log "you can use rebar to build OTP-compliant apps". So its a build tool? Not until "Basic Usage" do I find out in writing what rebar potentially is:

"Rebar3 comes with templates for creating applications, library applications (with no start/2), releases and plugins"

So it's a project templating tool? Does "comes with" mean it has more functionality than just templating? On the sidebar I see some terms that lead me to believe it can do more than that but I would love something on the landing page that told me exactly what the tool's purpose or function is.

From my understanding it is a tool similar to Leiningen?

Yes, pretty much. I think we went short on the rebar3 descriptions just because of how known 'rebar' is as a household name. Go to almost any Erlang project, and you'll find 'rebar' or some variant.

Rebar3 is the next version of it.

But you're entirely right, the page and documentation is lacking to people unfamiliar with the Erlang/OTP ecosystem.