Berachos, Chapter 6, Mishnah 7. This mishnah teaches the concept of 'ikar and tafel' (primary and secondary): the berachah is determined by the main food that a person wishes to eat, while the item that accompanies it - the tafel - is covered by the berachah made on the ikar.
The words of the mishnah:
"Heivi'u lefanav maliach batechilah u'pas imo" - a salted food was brought before him (salted fish or salted olives), and it is the main food before him, and together with it bread was brought.
Ordinarily bread is always considered the ikar. How, then, can there be an exceptional case in which the bread becomes tafel?
The Gemara explains that this refers to a person who ate a large quantity of very sweet fruit that day, and now he needs a salty food to remove the sweetness. The bread here does not come for the sake of eating it in its own right, but rather to enable him to handle the sharpness of the salty food, and therefore it is secondary to it.
In this case he makes a berachah on the salted food alone, according to the type of food:
Olives - the berachah of "borei pri ha'etz".
Fish - the berachah of "shehakol nihyah bidvaro".
And there is no need to make a berachah on the bread, "shehapas tefeilah lo" - for the bread is secondary to it.
The approach of Rashi:
Rashi explains differently what the "maliach" of the mishnah is: in his view it refers to a special fruit that they used to grow in the region of the Kinneret. In that area these fruits were extremely important and significant, to the point that they were considered even more important than the bread itself.
The underlying rule:
The mishnah concludes with a rule: "Zeh haklal: kol shehu ikar v'imo tefeilah, mevarech al ha'ikar u'poter es hatefeilah" - this is the rule: whenever there is a primary food accompanied by a secondary food, one makes a berachah on the primary food and thereby exempts the secondary one. The berachah on the ikar covers the entire berachah that would have been required for the secondary food.
In summary: in this mishnah we learned the principle of 'ikar and tafel'. Even though bread is generally considered the ikar, there are situations in which it comes only to serve the other food - such as someone who ate sweet fruits and needs something salty, and the bread comes to soften its sharpness - and then he makes a berachah on the salted food and thereby exempts the bread. Rashi explains that the "maliach" is an important fruit that grew around the Kinneret, which was considered even more important than bread. The rule that emerges from here: one who makes a berachah on the ikar exempts the tafel from a berachah.