What is the first rebellion in Animal Farm about (causes and what it was)?

If by the first rebellion, you mean when the animals
rebelled against Farmer Jones, the cause was a build-up of years of mistreatment of the
animals capped by an extended period of starvation where Jones preferred to get drunk
and be lazy to feeding his animals.

Old Major (who
symbolizes Karl Marx) planted the seed of rebellion a good deal of time earlier when he
presented his "dream" of a world where all animals were equal to the other animals. In
his speech, Old Major chronicled how the humans didn't produce, yet took the fruits of
the animal labor. He also reminded the animals of the cruelty of man that he would steal
the animal's children and slaughter the animals when they were too old to work for him
anymore. The rebellion song, "Beasts of England" riled the animals up to think
revolt...which they finally did on that fateful night when Jones refused to feed
them.

So "what it was" about was the animals banding
together, deciding that they no longer wanted to be controlled by an outside
entity...and rising up and defeating the current government...later replacing it with
their form of citzens-as-equals government called "animalism".