Knickerbockers are a type of pants that are shorter than regular pants and usually end just below the knee. They are sometimes referred to as knee pants or breeches.
Imagine wearing really long pants that go all the way down to your ankles. Now imagine cutting off the bottom part of those pants so that they are a little bit shorter, stopping at your knee. That's kind of what knickerbockers look like.
Knickerbockers were really popular a long time ago, especially back in the 1800s. People used to wear them when they played sports like golf, baseball, or tennis.
Nowadays, knickerbockers aren't as common, but you might still see people wearing them for things like costume parties, or if they're really into vintage or historical clothing.