Pesachim, Chapter 1, Mishnah 5. In the previous Mishnah we learned Rabbi Yehudah's opinion: a person may eat chametz on the eve of Pesach throughout the fourth hour of the day - on a day that runs from six to six, until ten in the morning. During the fifth hour we "suspend," and the term "suspension" is like a scale: the chametz is not eaten, but there is no obligation to burn it yet - it is still permitted to derive benefit from it and even to sell it. The burning is done at the beginning of the sixth hour.
"Ve'od amar Rabbi Yehudah" - the sign of the two loaves:
In our Mishnah, Rabbi Yehudah adds a further detail about how the problem of determining the time on the eve of Pesach was handled in the days of the Temple: "shtei challos shel todah pesulos munachos al gag ha'itztaba" - two disqualified loaves of a thanksgiving offering were placed on the roof of the colonnade.
The thanksgiving offering was a peace offering: an animal was brought, and with it forty loaves - ten of which were loaves of chametz. It is worth noting that everything containing grain in the Temple was unleavened; apart from the two loaves of Shavuos and the ten loaves of the thanksgiving offering, there was no chametz in the Temple at all, and everything else was fit for Pesach all year round.
Two of the ten chametz loaves of a particular offering, which had become disqualified, were placed on the eve of Pesach on the roof of the colonnade - a kind of high balcony in the Temple - so that they would be visible to all the people. In this way the two loaves served as a sign of the time:
"Kol zman shemunachos" - as long as both of them were lying up there, "kol ha'am ochlin" - all the people would eat: everyone knew that it was still permitted to eat chametz, meaning throughout the fourth hour.
"Nitlah achas" - once one of the two loaves was removed, they knew that the time of "suspension" had arrived: "lo ochlin velo sorfin" - the chametz is not eaten, but there is no obligation to burn it and it is still permitted to derive benefit from it. This is Rabbi Yehudah's opinion in the previous Mishnah, during the fifth hour.
"Nitlu shteihen" - once both loaves were removed, this was a sign that the time to destroy the chametz had arrived, and therefore "hischilu kol ha'am sorfin" - all the people began to burn it.
The opinion of Rabban Gamliel:
Rabban Gamliel raises a point that is really part of the dispute in the previous Mishnah, regarding exactly how late chametz may be eaten:
"Chulin ne'echal kol arba" - ordinary chametz, which is eaten by any Jew and not by a kohen alone, may be eaten throughout the fourth hour.
"U'terumah kol chamesh" - Terumah, which we do not wish to burn unless we must, since burning it is forbidden except when there is no alternative, may be eaten throughout the fifth hour, until eleven o'clock.
"Vesorfin bischilas shesh" - when the fifth hour ends, at the beginning of the sixth hour, the chametz is burned.
In summary: in this Mishnah Rabbi Yehudah adds how the time was determined in practice in the Temple - two disqualified thanksgiving loaves were placed on the roof of the colonnade, and their removal, one after the other, signaled the people's move from eating to suspension and from suspension to burning. Rabban Gamliel, by contrast, distinguishes between ordinary chametz, eaten throughout the fourth hour, and Terumah, eaten throughout the fifth hour, with the burning of the chametz at the beginning of the sixth.