Holy Light Monastery is a small settlement which is inhabited by the Apostles of the Holy Light. It has only one interior section.
Brother Gerard guards the entrance. In the partially collapsed wooden building behind him are two radiation traps which can be activated with a button on a stand nearby. In a corner of the building is a cellar door which leads inside.
Eternal Light Monastery
The inside, named "Eternal Light Monastery", features a small chapel with some benches where Mother Curie III preaches, with a book shelf off to the side of it. There are three beds for the cult followers, further downstairs are some radiation traps, a fridge, a table, a radio, two boxes of holy water, and two non-hostile feral ghouls. As well as Atom's Champion and Sun of Atom (which can also go upstairs but are most often encountered downstairs).
This location only appears once Broken Steel is installed.
The Monastery (and the house above) contains radiation traps (4 all together), as the name implies they 'trap' and concentrate radiation by capturing local particles in a field. However, these traps cannot be triggered by anything other than intentionally activating them. The traps are useful to increase the player radiation level in order to get inside the Monastery. These traps can be disabled.
If you travel to the Monastery before you finish the quest Take it Back, the building will look the same as it did without the add-on, but an inaccessible cellar door will be there.