A precautionary measure at the table (harchaka — lest one come to eat them together), and the distinguishing signs (heker) that remove it (Shulchan Aruch, Yoreh De'ah 88 — 2 seifim)
אֲפִלּוּ בְּשַׂר חַיָּה וָעוֹף אָסוּר לְהַעֲלוֹתוֹ עַל שֻׁלְחָן שֶׁאוֹכֵל עָלָיו גְּבִינָה, שֶׁלֹּא יָבֹא לְאָכְלָם יַחַד. אֲבָל בְּשֻׁלְחָן שֶׁסּוֹדֵר עָלָיו הַתַּבְשִׁיל — מֻתָּר לִתֵּן זֶה בְּצַד זֶה.
The prohibition of the shared table: Even the meat of a wild animal or of fowl may not be placed on a table where cheese is eaten, lest one come to eat them together. But on a table on which one merely sets out the dishes (before serving them), it is permitted to place one beside the other.
Shulchan Aruch, Yoreh De'ah 88:1LEVEL 01
Hebrew text of the 2 seifim of the Mechaber and the Rama with a fluent English translation. The reason for the prohibition (harchaka / shema yavo le'okhlam yachad), the key concepts (heker, makirim, akhsanaim), an introduction to the Shach and the Taz, and modern practical cases.
📖 StudyLEVEL 02
In-depth pilpul: the sugya of Chullin, גזירה לגזירה, מחלוקת הש״ך והט״ז עם הרש״ל והב״ח, גדר ההיכר, האם האיסור בשאר איסורין, חקירות ונפקא מינות — all in lashon ha-kodesh.
📖 StudyLEVEL 03
Comparative tables, golden rules, classic pitfalls (who knows whom, which heker counts), mnemonic and quick memorization of the 2 seifim.
📖 StudyLEVEL 04
The practical halacha according to the Shach, Taz, Pri Megadim and Pithei Teshuva, then the Sephardic schools of psak (Yabia Omer, Yalkut Yossef, Or LeTzion) and the Ashkenazic ones (Iggrot Moshe). Note: the Shulchan Aruch HaRav does not treat this siman — a level of psak, not "Daat HaRav."
📖 StudyThe Shulchan Aruch (YD 88:1) forbids it lest one come to eat them together (שלא יבא לאכלם יחד). It is a precautionary measure of the Sages: even the meat of fowl or of a wild animal (whose mixture with milk is only rabbinic) is not placed on a table where cheese is eaten. The Shach (s.k. 1) notes that the rule applies in both directions — cheese on a meat table as well. For practical halacha, consult your Rav.
It depends. Seif 2 distinguishes: if the diners know one another (מכירים זה את זה), it is forbidden, even if they are sitting next to each other. If they do not know one another (אכסנאים), it is permitted. And even among people who know one another, the prohibition can be removed by placing a distinguishing sign (heker) between them. For halacha le-ma'aseh, consult your Rav.
A heker is a visible reminder that prompts one to be careful. The Mechaber (seif 2) gives several: each person eats on his own tablecloth; a loaf of bread is placed between them (provided it is not eaten); a drinking utensil that does not normally belong on the table; a menorah or another unusual object; and the custom of giving each person his own salt-cellar. For halacha le-ma'aseh, consult your Rav.