An oxer is a horse jump with two rails set a distance apart. Sometimes there is "filler" in the oxer, such as brush. The rails may be the same height or different heights, but the front rail is never higher than the back rail, as this messes with a horse's vision and causes them to knock the fence down. A lower front rail is considered more inviting.
In the UK, an oxer where the two rails are at the same height is called a "parallel."
An oxer with three rails is a "triple bar."
The horse is jumping a parallel rails or parallel oxer, with a regular ascending oxer in the background. Image source SGBailey via Wikimedia Commons.