Railsドキュメント

label(オブジェクト名, メソッド名 [, ラベル配下のコンテンツ, オプション])

f.label(メソッド名 [, ラベル配下のコンテンツ, オプション])

label(:post, :cost)
# <label for="post_cost">Total cost</label>

label(:post, :title, "A short title")
# <label for="post_title">A short title</label>

label(:post, :title, "A short title", class: "title_label")
# <label for="post_title" class="title_label">A short title</label>

label(:post, :terms) do
  raw('Accept <a href="/terms">Terms</a>.')
end
# <label for="post_terms">Accept <a href="/terms">Terms</a>.</label>

f.label :name
# <label for="page_name">Name</label>

f.label :name, @page.name
# <label for="page_name">abc</label>

f.label :name, '', class: 'page_name'
# <label class="page_name" for="page_name">Name</label>