This is a general counter for two unspecified items or "two pieces." It is used when counting objects that do not have a specific counter. For example, you can say "two apples" or "two questions" using this word.

形(かたち)や種類(しゅるい)を問(と)わず、物(もの)を「二(ふた)つ」数(かぞ)えるときに使(つか)います。例(たと)えば、リンゴ二(ふた)つ、質問(しつもん)二(ふた)つなど。