Tribulation is basically something that occurs because of one of many reasons: reaching high level cultivation bottlenecks(depending on law when), reaching old ages, divine cultivation upset followers, optional ones from reaching high attainment, and yeah ones from being a intelligent humanoid animal person trying to become a 'human'. If you want to be a catgirl(girl with catears) instead of a cat head on a human body then you gotta go through one.
Lore-wise they are the heavens striking you down for going against the natural order...
Practical in-game-wise they manifest as a lightning tornado thing that shoot lighting at your cultivator undergoing it and anything nearby the cultivatior. They have elements associated with them so can be weakened by taking them in elemental areas that are against them, or strengthened by elements that strengthen them. Some tribulations can have multiple elements, and thus have parts. As you undergo it it shows you the qi left in the tribulation and if you survive that, that part is over (assuming its a basic one-part one, just doing this once means its over). This is as far as i can tell, pretty much the same thing as fighting a enemy cultivator, the tribulation has qi, you have qi, you attack each other, it expends qi to attack you, and you decrease its qi by attacking it... your barrier skill therefore matters ALOT as does your qi repleshment, so taking a shield pill and something like spirit stones(and he improved versions preferably like spirit crystals) that give qi-repleshment rates help TONS.
The Animal-become-human tribulation is fairly difficult since its a 5part one, but get to gold core first, eat a shield pill and spirit crystal, maybe use something that offers '5 element protection' or increases qi protection/shield, and you CAN beat it, even with a shitty gold core cultivator, will cost you a lot of resources though.
Fair warning: DO NOT undergo tribulation anywhere that your people might walk by, as they will get hit by lightning and die... or near anything you don't want lit on fire by said lightning.