Before us is Mishnah 1 in the third chapter of Maseches Eruvin, which opens with a discussion of which types of food may be used to create an eruv. Two types of eruv are discussed here: eruv chatzeiros, which is made in order to permit carrying in a courtyard, and shituf mevo'os - when several courtyards open into one alleyway, and all of them must join together in a shituf by placing food in common, in order to permit carrying in the alleyway.
"Bakol me'arvin u'mishtatfin":
"Bakol me'arvin u'mishtatfin" - with any type of food one may create an eruv chatzeiros or a shituf mevo'os. The Rishonim disagree about the scope of this rule:
Some Rishonim understand that the rule "bakol me'arvin" refers specifically to eruvei techumin, and does not include eruvei chatzeiros, since eruvei chatzeiros are made only with bread and not with any other type of food.
Other Rishonim understand that eruvei chatzeiros are included as well, and not only eruvei techumin.
"Chutz min hamayim u'min hamelach":
"Chutz min hamayim u'min hamelach" - water and salt are the exception, because they are not considered food and they are not nourishing, and the eruv must be something nourishing. The Gemara notes that although the Mishnah listed only these as exceptions, there are additional items included in that same exception, such as mushrooms and truffles, which likewise do not have the status of food.
Extending the discussion to other areas of halachah:
Since the Mishnah is dealing with the question of which foods fall under the category of nourishment, it extends the discussion to two additional areas of halachah:
"Hakol nikach b'kesef ma'aser, chutz min hamayim u'min hamelach" - one who brings up his ma'aser sheni money to Yerushalayim may use it only for food; therefore any type of food may be bought with ma'aser sheni money, except for water and salt.
"Hanoder min hamazon, mutar b'mayim uv'melach" - one who takes a vow not to eat food, even though his vow includes every type of food, is permitted water and salt, since they are not included in the term "food."
"Me'arvin l'nazir b'yayin ul'Yisrael b'Terumah":
"Me'arvin l'nazir b'yayin ul'Yisrael b'Terumah" - one may make an eruv for a nazir with wine, even though he himself is forbidden to drink it, and for a Yisrael who is not a kohen with Terumah, even though he cannot eat it. The reason: since these foods are fit to be eaten by others, that is enough. Rabbi Shimon, however, disagrees and says: "B'chullin" - one who is not a kohen may make an eruv only with chullin, with something that he himself can eat, and not with something forbidden to him, such as Terumah.
Placing an eruv in a place the kohen cannot enter:
The Mishnah now presents a concept called beis haperas: a field that contained a grave, and the grave was plowed over with a plow. The concern is that the plow scattered bones from the grave throughout the field, and even a bone the size of a barley grain conveys tumah through contact. Therefore there is a concern that a bone may be found in the field up to a hundred amos in every direction, and a kohen may not enter a beis haperas.
And since the kohen cannot enter there, the question arises whether the eruv may be placed there. The Mishnah rules that it may, in its words: "V'lo kohen b'veis haperas". How is this possible, if the kohen cannot reach the eruv? The Gemara explains that there is a way for the kohen to enter a beis haperas: if he blows away the dust in front of him and walks with great care, checking that there is no bone there, he is permitted to walk there. It turns out, then, that he is able to reach the eruv placed in the beis haperas.
Rabbi Yehudah goes even further and says: "Afilu b'veis hakevaros" - a kohen may place his eruv even in a cemetery itself, "midshiyachol leilech lachutz v'lachzor". The reason: he can actually enter there as well, by creating a chatzitzah - a partition between himself and the graves, which blocks the tumah from reaching him. He will therefore be able to enter, take the eruv and eat it if he needs to, and for that reason the eruv is valid even for a kohen in a cemetery.
In summary: In this Mishnah we learned that an eruv and a shituf may be made with any food, except for water and salt, which do not fall under the category of nourishment (and likewise mushrooms and truffles), and from this halachah the discussion was extended to ma'aser sheni money and to one who vows not to eat food. We also learned that an eruv may be made for a nazir with wine and for a Yisrael with Terumah, because the food is fit for others, while Rabbi Shimon disagrees and requires chullin. Finally, we examined placing an eruv in a place that is difficult for a kohen to enter: in a beis haperas - since he is able to enter by blowing away the dust and checking; and according to Rabbi Yehudah even in a cemetery - since he is able to enter by means of a chatzitzah that blocks the tumah.