Remember how back when you were very little, people used to wear their pants super low on their hips? Well, when we talk about "2000s in fashion," we're talking about the stuff that people wore around that time.
One big thing that people did in the 2000s was wear really tight clothes. Women would wear tops that were called "baby doll" tops because they were stretchy and fit really snugly around the bust before flaring out a bit around the midsection. Another kind of popular top was called a "halter," which is a top that ties behind your neck and leaves your shoulders bare. It was usually tight around the chest area too.
For guys, a popular style was baggy clothes. This means that their pants and shirts were really loose and not tight like girls' clothing. Guys would often wear baggy jeans that hung low on their hips and had really wide legs.
Another trend that was really popular was something called a "low-rise" pant. These are pants that sit really low on your hips, often leaving your underwear showing (this is where the people wearing the really tight underwear and showing the waistband came from). Some people would wear shorts that were even lower than their pants, and they were called "booty shorts."
There were also different shoe styles that people wore in the 2000s. Girls often wore Uggs, which are really comfy boots that look like they're made of fluffy sheepskin. Everyone wore shoes called "flip-flops" a lot too. These are sandals that have a thong between your big toe and the rest of your toes to hold them on your feet.
One thing that was really popular for both boys and girls was a style called graphic tees. This means t-shirts with big, funky designs on them. People would often wear them with a lot of jewelry, like necklaces and bracelets.
Overall, people in the 2000s tended to dress a lot more casually than they did in earlier decades. They liked to wear really comfortable shoes and clothes that they could move around in easily.