Routing In React, what is this `:tab` thing

Why does the ionic cli create a blank app in react with a route containing path="/:tab(home)"

<Route path="/:tab(home)" component={Home} exact={true} />

When path="/home" works just fine?

 <Route path="/home" component={Home} exact={true} />

I cannot find nothing in the documentation using this :tab format and I have written an app with it using tabs and it seems to be working fine