There are two instances in this week's Parshas that emphasize the importance of keeping one's word. The first section discusses the nullification of vows. The Torah begins this section by telling us to be careful not to לא יחל דברו, that one should not make his words חולין, or profane his words. The Torah goes on to say, ככל היוצא מפיו יעשה, that one should be careful to watch what comes out of one's mouth.
This certainly applies when one makes a vow. In קהלת it is written, טוב שלא תידור משתדור ולא תשלם, that it is better not to make a vow than to vow and not fulfill it.
The second part of the Parsha deals with the two and a half tribes who wished to stay on the other side of the Jordan. Moshe makes a condition with them that they can keep that land if they keep their word and fight with the rest of the nation.
So we see the importance of being careful to keep one's word and fulfill whatever a person says he's going to do. Words are very powerful and are capable of doing great harm. This is why the Mishna says, סיג לחכמה שתיקה, a fence for wisdom is silence. Shabbat Shalom