I heard of Hypoxic Ischemic Encephalopathy relating to pregnancy, but what is it and how it is caused?

hy·pox·i·a (hī-pŏk’sē-ə, hĭ-) n. Deficiency in the amount of oxygen reaching body tissues.
hy·pox’ic adj.

is⋅che⋅mi⋅a   /ɪˈskimiə/ –noun Pathology. local deficiency of blood supply produced by vasoconstriction or local obstacles to the arterial flow.

en⋅ceph⋅a⋅lop⋅a⋅thy   /ɛnˌsɛfəˈlɒpəθi/ –noun Psychiatry. any brain disease.

So…: not getting enough oxygen to the brain due to a restriction in blood flow.

The question would be what causes the restriction in blood flow? Traumatic injury? Arterial clogging (plaque, cholesterol, etc.)?

HIE is a fetal complication, not a maternal complication. You can read more about it here.