There is a reasonably good explanation of Washington’s advice about factions and political parties being divisive and repressive at wiki. Washington worked for union rather than division so he was of course against ‘alternate despotism’ of parties that lead to permanent despotism. He said governments have tried to repress parties and felt it was a good idea. Factions also were bad in his opinion as the nation needed to stand together then against foreign enemies. Washington was right on most political ideas being a realist.
Apparently two parties had arisen in his day; one supported France and the other wanted to align with Britain during the French revolution. Royals all over Europe worried about the French example. Factions allied with external cohorts were dangerous. One sees Democrats aligning today with foreign interests in Ukraine and Latin America regarding war and illegal immigration of an external proletariat with quite a divide of opinion about building a border security wall. Republicans tend to have an opposite opinion regarding a Mexican and beyond external proletariat that might work a creeping annexation if they weren’t stopped. The two parties are also factions.
“Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote” – Benjamin Franklin
Washington also thought parties and factions tended to alienate themselves from the people. He was concerned about some strong individual or even dictator or monarch arising and the nation had spent a lot on revolting against that. Washington himself voluntarily limited any movement that would make a king of himself. Some elites tend to support rule by elites or even just one and Washington was the nation’s defender fighting and advising in support of republican populism.
G.W. had republican Rome as an example that morphed into an imperial empire with Caesar at the top. Today the rich plutocrats and the Congress are too closely aligned. Many Americans don’t feel too well represented in Congress. Washington also warned about the military industrial complex becoming too powerful like Eisenhower had (just the military part though because industry didn’t exist in the same way in the late 18th century). Americans pay too little attention to George Washington’s common sense these days.