There is a famous Halachic concept called "Shome’a Ke’one," which allows a person to fulfill his obligation to recite a certain text by listening to somebody else’s recitation. On Shabbat, for example, it is customary for one person to recite Kiddush, and the others at the table fulfill their obligation by listening to that recitation and reciting "Amen."
A number of conditions must be met for this mechanism of "Shome’a Ke’one" to work. For one thing, both parties must have Kavana (intention) that the recitation should fulfill the obligation of the listeners. The one reciting Kiddush must have in mind that the others will fulfill their requirement by listening to his Kiddush, and they, too, must have this in mind as they listen to the Kiddush.
The Shulhan Aruch (Orah Haim 193:1) adds another important condition, of which many people are unaware. He writes (listen to audio recording for precise citation) that the listener, who fulfills his obligation by hearing the recitation, must understand what is being said. Based on the comments of a number of earlier authorities (Tosafot, Rabbenu Yona and the Rosh, Berachot 45), the Shulhan Aruch maintains that the system of "Shome’a Ke’one" requires the listener’s comprehension of the recited text. Often, people at the table on Friday night do not necessarily understand the words of Kiddush. It would therefore be proper before Kiddush to briefly explain the words of Kiddush so that the listeners will understand what is being said.
Nevertheless, if one listened to Kiddush (or another required recitation) and did not understand what was recited, he has Be’di’abad (after the fact) fulfilled his obligation. This is the ruling of the Mishna Berura (commentary to the Shulhan Aruch by Rav Yisrael Meir Kagan, 1839-1933), who writes that in such a case one may rely on those authorities who do not require the listener to understand the text. This is also the ruling of Rabbi Moshe Halevi (Israel, 1961-2001), in his work Birkat Hashem (vol. 1, p. 171; listen to audio recording for precise citation).
It must be emphasized that this refers only to the recitation of a Hebrew text. If a person recited a Beracha or Kiddush in a different language, then according to all views the listener fulfills his obligation only if he understands what was recited. It is only regarding a Hebrew recitation that some authorities allow the listener to fulfill his obligation even if he does not understand the language.
Summary: Before one recites Kiddush on Friday night, he should make sure that all those who will be fulfilling their obligation by listening to his recitation have a basic understanding of the words of Kiddush. Nevertheless, if someone did not understand, he nevertheless fulfills his obligation by listening to Kiddush. This applies as well to other situations where one fulfills his obligation to recite a certain text by listening to its recitation by somebody else.
