A Barrow Hill is a Thaumcraft created dungeon that appears on the surface of the world. It is disguised to look like an small ordinary hill, but it has an iron grate on one side which reveals its position.
These locations can be quite dangerous to enter unprepared. It is very dark inside and has two monster spawners so there is almost always going to be a small horde of enemies inside. Entrance does not, however, come with no reward as each Barrow Hill does also contain three chests. These chests have a high chance to spawn Knowledge Fragments and Thaumium ingots and tools, that includes the tier-2 items as well.
The fact that these locations always contain two monster spawners, one zombie and one skeleton, as well as seemingly endless supply of wisps make these places good place to farm for experience. You can easily craft a grinder on the lower floor for the skeletons and zombies, and simple run "upstairs" to kill whatever wisps may have spawned.