It is a Talmudic debate whether commandments require kavana—intention—to be considered fulfilled. One opinion states that if a person does not know what they are doing when performing a mitzvah, they have not fulfilled the mitzvah. The other opinion maintains that a person can fulfill a mitzvah even if they do not understand or know what they are doing (of course, the second opinion agrees that it is much better to have kavana, but even without it, according to this opinion, the commandment is still fulfilled).
The halakhic conclusion – Jewish legal – is a kind of combination of both opinions: the person must know at the time of observing a precept (e.g., when affixing a Mezuzah) that what he is doing is a Divine precept, a commandment from God, the King of all kings, the Holy One, blessed be He.
Of course, it is better to know the details of the Mitzvah that the person is doing, both from the halakhic point of view - the laws related to that Mitzvah - and from the spiritual point of view - the meaning of the Mitzvah, but they are not an exclusive requirement to be considered as having fulfilled the Mitzvah.
However, there are some exceptions.
For example, the mitzvah of tefillin (which technically consists of two separate precepts, head tefillin and arm tefillin) requires more than just the general kavana of being a divine commandment. Jewish law states that a person must understand that head tefillin symbolizes the surrender of the intellect to God, while arm tefillin symbolizes the surrender of the heart to God (where feelings reside).
Another exception to the aforementioned rule is the mitzvah of the Sukkah. This mitzvah consists (in short) of living in a sukkah for the seven (or eight) days of Sukkot. However, the law specifies that it is not enough to simply sit in the sukkah and enjoy the fresh air; rather, the person must know (have kavana) that they are sitting in the sukkah because God surrounded the Jewish people with His clouds of glory when He brought them out of Egypt.
It should be noted that if a person did not think about these matters while putting on Tefillin or while in the Sukkah, they did not fulfill the Mitzvah...