Our Mishnah deals with certain activities a person carries out in his field - activities that might appear as though his intention is to improve and prepare the field in anticipation of Shemittah. The question is what measure makes it clear that this is not his intention.
The words of the Mishnah:
"Lo yiftach adam machtzav bat'chilah l'toch sadehu, ad she'yehu bo shalosh morviyos, she'hen shalosh al shalosh al rum shalosh"
"Lo yiftach adam machtzav bat'chilah l'toch sadehu" - a person may not open and begin digging a stone quarry in his field.
"ad she'yehu bo shalosh morviyos" - unless there is enough stone in that quarry for three morviyos, that is, three rows.
"she'hen shalosh al shalosh al rum shalosh" - three amos by three amos, at a height of three amos: a cube whose every side measures three amos.
The measure of such a cube is twenty seven stones, each one a cubic amah. This is the quantity required in order for it to be clear that his purpose in taking the stones is for building needs, and not in order to improve the field for planting.
In summary: we learned that a person may not open a new quarry in his field unless it contains the measure of three morviyos - three amos by three amos at a height of three amos, which amounts to twenty seven stones of a cubic amah each. This measure demonstrates that his intention is for building needs, and that he is not coming to prepare his field for planting.