This is a general counter for eight unspecified items or "eight pieces." Use this word when counting general objects that do not have a specific counter, for example, "eight cookies" or "eight stars."

形(かたち)や種類(しゅるい)を問(と)わず、物(もの)を「八(やっ)つ」数(かぞ)えるときに使(つか)います。クッキー八(やっ)つ、星(ほし)八(やっ)つなど。