This is a general counter for one unspecified item or "one piece." It is used when counting objects that do not have a specific counter associated with them, like "one apple" or "one question."

形(かたち)や種類(しゅるい)を問(と)わず、物(もの)を「一つ(ひとつ)」数(かぞ)えるときに使(つか)います。例(たと)えば、リンゴ一つ(ひとつ)、質問(しつもん)一つ(ひとつ)など。