A jumper has different meanings depending on where you are:

1. In American English, a jumper is a sleeveless dress worn over a shirt or blouse. You might see students wearing jumpers as part of their school uniforms, typically a simple dress in a solid color with a shirt underneath. Think of it like overalls in dress form: it covers your front and back but leaves your arms bare, so you need to wear something with sleeves underneath.
2. In British English, a jumper is what Americans call a sweater: a knitted garment with sleeves that you pull over your head to stay warm. When British children talk about wearing their jumpers to school, they mean the same thing American kids mean when they say sweaters.
3. A jumper can also be a person or animal that jumps. A horse that's good at jumping over fences is called a jumper. An athlete who competes in the high jump or long jump is also a jumper.