This is a shorthand syntax. From the docs:
Every partial also has a local variable with the same name as the partial (minus the underscore). You can pass an object in to this local variable via the :object option:
<%= render partial: "customer", object: @new_customer %>
Within the customer partial, the customer variable will refer to @new_customer from the parent view.
If you have an instance of a model to render into a partial, you can use a shorthand syntax:
<%= render @customer %>
Assuming that the @customer instance variable contains an instance of the Customer model, this will use _customer.html.erb to render it and will pass the local variable customer into the partial which will refer to the @customer instance variable in the parent view.