Every member of the Jewish nation is obligated to read the Megillah on the day of Purim. One must read it during the night and once again the next day, as the verse states, “My G-d, I call out to you during the day and you do not answer; during the night I have no rest.” This verse is written in the chapter of Tehillim (Psalms) called “Lamnatze’ach al Ayelet Ha’Shachar” and the Gemara in Yoma (29a) refers to Queen Esther as the “Ayelet Ha’Shachar”. Therefore, we derive laws from this chapter regarding the Megillah which was co-authored by Mordechai and Queen Esther since they were the ones to institute the reading of the Megillah on the day of Purim.
However, since not everyone is an expert in Megillah reading along with the fact that not everyone owns a kosher Megillah written on parchment, we customarily fulfill our obligation of Megillah reading through the law of “one who hears is tantamount to one who recites.” This means that one who hears another person reading from a kosher Megillah fulfills one’s obligation and it is as though one has read the Megillah himself. Therefore, when the Chazzan reads from a kosher Megillah while having in mind to read on behalf of the congregation and the congregation has in mind to fulfill their own obligation as well by listening, it is considered as if they have actually read the Megillah on their own.
During the time when the Megillah is being read, one may not interrupt by speaking at all until after the blessing at the conclusion of the Megillah reading has been recited. One who did indeed speak during Megillah reading while the Chazzan carries on with the reading of the Megillah does not fulfill his obligation and his status is like one who missed a portion of the Megillah reading, as will be explained later on.
One should listen carefully to the reading of the Megillah and one should exercise special care not to miss hearing even one word from the reading of the Megillah, for according to many Poskim (including the Rashba, Ran, and others), if one misses hearing even one word of the Megillah, one has not fulfilled one’s obligation.
However, if it happens that one missed hearing a few words of the Megillah because of noise and the like, one may read those words from within the Megillah one is holding, even if it is not a kosher Megillah; rather, even if it is a printed one, one should quickly read the words one missed until one reaches the place where the Chazzan is currently reading, at which point one should once again remain silent and listen to the Chazzan’s reading. (One will fulfill one’s obligation as long as one has heard most of the Megillah reading from the Chazzan and has only read a minority of it on his own from the printed Megillah in one’s hands).