The entire Mishna states as follows:
“Shimon [the son of Rabban Gamliel], says: ‘All of my days I have grown in the midst of the Sages and I have found nothing better for the body than silence, and the expounding of Torah [teachings] is not the main thing, rather, the action [is the main thing]. All those who increase words bring sin.”
This entire Mishna discourages excess speech, as well as speaking when it potentially leads to negative results.
The Kihati, quoting Midrash Shmuel, explains this Mishna as follows:
For matters relating to the needs of the body, silence in the best attribute, however, in relation to the soul, certain speech, such as Torah leaning and Tefillah (prayer), is beneficial. If people are transgressing a certain part of the Torah or are lax in their performance of the mitzvos, he should not spend too much time exhorting others to repent and strengthen their practice of the mitzvos in a situation where such exhortation is at the expense of setting an example for the people by taking an action to do that which is necessary in order to fix the breach in the ways of the Torah. All those who speak at length, informing people of their transgressions without taking actions to encourage the cessation of these transgressions, can lead to an increase in sin, for he makes the transgressors into those who intentionally transgress the Torah, and, as a result, can increase the spiritual prosecution directed against the Jewish People.
According to the Bartenura, if one expounds the Torah and does not perform the mitzvos, it is better for him to be silent and not expound the Torah.
Similarly, we learn later in Pirkei Avos (Chapter 3; Mishna 17) that one whose performance of Torah is negligible and who learns a significant amount of Torah, is comparable to a tree with many leaves and few roots, if the wind comes, it knocks that tree over. However, one who performs the mitzvos, much or all of his Torah learning is directed to providing him the knowledge to perform the mitzvos, he is comparable to a tree which has many roots and few leaves, that tree being able to stand up to all of the winds of the world.
 The Sages are compared to “the cedars of Lebanon”, while we are compared to “the moss on the wall”, to exemplify the great difference in the spiritual stature between the rabbis of the early generations, such as the scholars quoted in the Mishna and Gemara, and the people of our generation.