A question about rails view code

I am new to rails. I read some examples and don't understand rails erb
syntax. There is a piece of code I am not familiar:

<% form_for(@bay_info) do |f| %>
  <%= f.error_messages %>

  <p>
    <%= f.label :HostName %><br />
    <%= f.text_field :HostName %>
  </p>
...

I wander what f mean. What is its type? How can I find its definition?

thanks.

I am new to rails. I read some examples and don't understand rails erb
syntax. There is a piece of code I am not familiar:

<% form_for(@bay_info) do |f| %>
<%= f.error_messages %>

f is a form builder. If my memory is correct the docs for form_for
should have some details.

Fred