Why the instance variables can be in form_fo in HTML?

This is my code in

users_controller.rb

def index
 @users = User.paginate(page: params[:page])
end

def show
  @user = User.find_by id: params[:id]
  return if @user
  flash[:danger] = t("controllers.users_controller.failed")
  redirect_to signup_path
end

def new
 @user = User.new
end

def create
  @user = User.new user_params
  if @user.save
    log_in @user
    redirect_to @user
    flash[:success] = I18n.t("controllers.users_controller.success")
  else
    render :new
  end
end

And this ís my html: _form.html.erb

<%= form_for @user, url: yield(:url) do |f| %>
  <%= render "shared/error_messages" %>

  <%= f.label :name %>
  <%= f.text_field :name, class: "form-control" %>

  <%= f.label :email %>
  <%= f.email_field :email, class: "form-control" %>

  <%= f.label :password %>
  <%= f.password_field :password, class: "form-control" %>

  <%= f.label :password_confirmation, t(".confirm") %>
  <%= f.password_field :password_confirmation, class: "form-control" %>

  <%= f.submit yield(:button_text), class: "btn btn-primary" %>
<% end %>

And somebody help me know that why was the instance variable (@user) be in the form_for in html?