In the UK it describes the garment we, in the US, would call a 'pull-over sweater'.
Here, a 'jumper' is a very different sort of garment. It's a loose, sleeveless dress usually made of a heavy fabric such as wool or corduroy. The jumper is always worn over a turtleneck or a long sleeved blouse.
When made of velvet, the jumper is the dress of choice for young girls at winter holiday parties. It's reasonably warm. It's more comfortable than a standard party dress. It also looks much neater than a skirt and blouse.
Because it's so practical, I have no doubt that what we call a jumper is often used in the UK. However, I have no idea what it is called there.