This is an injury to the weight bearing surface under the heel bone. There is a highly specialised pad of fat here that acts as a natural shock absorber. This can become bruised either as a direct injury, for example a fall landing hard on the heel, or as an overuse injury from running on hard surfaces with inadequate footwear.
It feels like a bruise on the heel, which it is. The heel strike phase of walking is painful and you will feel the need to wear well cushioned shoes. You should try to rest the area whilst it recovers and ice can help if it is very sore.
If you think you have this condition it may be helpful to see Mr Willis-Owen. Whilst it will likely settle on its own with some rest, he will be able to assess why this has happened and help to correct any training errors and assess biomechanical problems to prevent recurrence.