A HDR image is a High Dynamic Range image created through the combination of photos taken at a range of high and low exposures. This allows more flexibility when it comes to adjusting the exposure of the image.
A standard image is limited in its ability when it comes to adjusting the exposure and similar attributes. Details within the image that are either solid white or solid black will only be that despite any changes in exposure. Unlike the human eye or a camera additional detail hidden in the dark can not be brought out by adjusting the exposure.
By taking photos at varying exposures and combining the images, we are able to achieve a photo which is capable of having its exposure freely adjusted. This means that when lowering or raising its exposure in software such as Maya or Photoshop areas that appear solid black or solid white at certain exposures will have more detail brought out when exposure is changed rather then them remaining solid.
HDR images are effective in 3D mediums because of this ability to adjust exposures. It allows for more realistic lighting of a scene and for the scenes lighting to be adjusted without losing the detail of the information being used to light it.