Lots of stuff; too much to name really. I guess if you're not a Pokemon fan anymore then it wouldn't spoil it for you if I went ahead and told you some important things that happened.
Ash ended up in the top 16 at the Johto League, and though he beat Gary, he was defeated by another one of his friendly rivals, Harrison. After that he went back to Pallet Town and learned of the new region, Hoenn. Misty's sisters won a trip around the world, and so Misty was left having to take care of the Cerulean Gym. Brock also had buisness to take care, and so Ash, Misty, and Brock all seperated.
Now Ash is left traveling towards Hoenn with only Pikachu. He left all of his other Pokemon with Professor Oak at his lab, and now he also has a new outfit. Pikachu ended up getting really sick along the way, and once they arrived at Hoenn, Ash had to find somewhere else besides the Pokemon Center to heal Pikachu, since Littleroot Town didn't have one. He ended up meeting Professor Birch, who later on at his lab figured out what the problem was with Pikachu; during an earlier fight with Team Rocket, they used a magnet to attract this metal thing wrapped around Pikachu, (long story) and while Pikachu was stuck on the magnet too much electricty was built up, or something like that. Anyways, while they were at the lab they met a new Trainer named May, who was ten. (Though.. she doesn't look it. o_o *cough*) Well, May ended up joining Ash and Pikachu on their journey, etc. etc. Brock rejoins after a few episodes, (With a new outfit also..) and later on while they're in Petalburg City, they meet May's family and her little brother, Max, joins them too. So, the journey continues...
If that didn't explain enough, then go somewhere like
Serebii.net for lots of episode summaries and pictures. ^_^; I don't have time to explain anything else that happened in the Johto region, if that's what you wanted to know. But most Johto episodes were sort of fillers anyway. The new ones are much more interesting, IMO. =P
