What conference does the pigs' meeting with the neighboring farmers symbolize?

If you're looking for a conference analogous to the pigs' meeting, the Tehran Conference is the best comparison. More generally, the meeting with the neighboring farmers represents the complete sellout of revolutionary Animalism by Napoleon. Previously, humans were regarded as the implacable enemy of the farm animals, to be comprehensively defeated in a life-or-death struggle. Yet now, as Napoleon shows the humans round Animal Farm, all that appears to have been forgotten. The pigs have learned to walk upright and are now virtually indistinguishable from the human farmers from which they fought to free themselves.
The greedy farmers—representing Western capitalists—admire the way that Napoleon has managed to get the animals on the farm to work harder but for less food. For his part, Napoleon expresses the hope that peaceful cooperation can be established with humans. He shows his goodwill by telling the farmers that all the old revolutionary customs have now been abolished. For instance, the animals will no longer be required to address each other as "comrade." For good measure, the animals will cease paying homage to Old Major, the originator of Animalism.
In keeping with the rest of the book, Orwell's intentions here are satirical. He's drawing our attention to the fact that the Soviet Union under Stalin became a much more exploitative society than the capitalist countries they sought to surpass. Whatever virtues Animalism—that is, Communism—may have had in principle, in practice they've led to a system of poverty and exploitation, which in many respects is indistinguishable from the worst features of capitalism.
For all the diplomatic words and surface bonhomie, it's clear that Napoleon and the human farmers still hate and distrust each other. But at the same time they've learned to come to some kind of arrangement which avoids the possibility of another all-out conflict. Orwell is alluding here to the uneasy accommodation between the Soviet Union and the West that characterized relations leading up to the Cold War.