The Miswah (מצוה - commandment) of Hafrashath Hallah (הפרשת חלה - separating a portion of the dough), was given to Benei Yisrael right after the sin of the Meraggelim (מרגלים - the 10 spies), when they went to the Land of Israel. When they returned, they told the nation that the land is a good land but with the type of people there, there was no chance of inheriting it. In other words, they made the people of Israel scared and caused them to start crying and to not want to go to this beautiful land that Haqqadosh Barukh Hu promised us.
There and then, Hashem punished the whole generation; all the men from the age 20 would die in the desert. At that point, the Children of Israel felt that perhaps there was no hope for them and that maybe they would never enter the Land of Israel. Right at that moment, Hashem gave them a special Miswah: the Miswah of Hafrashath Hallah. They were told that as soon as they come to the land, they will have a new Miswah that they will have to keep; the commandment of the separation of the bread (Hafrashath Hallah).
Through the giving of this Miswah, Benei Yisrael got a new sense of hope. They realized that by Hashem telling them that they would be obligated to do this Miswah as soon as they came to the Land of Israel, that He was obviously promising them that they would, in fact, enter the land of Israel. This gave them hope at this time of terrible distress.
Furthermore, there were other commandments that were given to the Children of Israel when they were in the desert, and Hashem told them that those מצות were also to be done only in the land of Israel. For example, giving Terumah, 1/10th of what we have to the Kohen. The instruction concerning the Bikkurim, when we see the first fruit or the first produce that grows, we also have to give as a present to the Kohen.
However, all these commandments came into effect 14 years after the Jews went to the Land of Israel. In other words, when they came to the land of Israel, they had to fight for the land of Israel and divide it among the different tribes. Therefore, these other commandments that were given to them to do in the Land of Israel started only 14 years after they actually went into the land of Israel. The only exception was the Miswah of Hafrashath Hallah, which came into effect as soon as Benei Yisrael entered the land of Israel.
We will look at the special significance attached to this Miswah in an upcoming Women's Corner.