Neurofibrillary tangles (which are technically described as 'aggregates of hyperphosphorylated tau protein') are commonly known as a primary marker of Alzheimer's diseaseplugin-autotooltip__plain plugin-autotooltip_bigAlzheimer's disease

"Alzheimer’s disease is a chronic progressive neuro-degenerative disorder characterised by three primary groups of symptoms. The first group (cognitive dysfunction) includes memory loss, language difficulties, and executive dysfunction (that is, loss of higher level planning and intellectual coordination skills). The second group comprises psychiatric symptoms and behavioural disturbances — for example, depression, hallucinations, delusions, agitation — collectively termed …. They are also prevalent in numerous other diseases - generally known as tauopathies. Little is known about their exact relationship to the different pathologies.

They are currently controversial because it's not known if they are causing disease, or simply a symptom of it.

Neurofibrillary tangles are formed by hyperphosphorylation of a microtubule-associated protein known as tau, causing it to aggregate, or group, in an insoluble form. (These aggregations of hyperphosphorylated tau protein are also referred to as PHF, or "paired helical filaments"). The precise mechanism of tangle formation is not completely understood, and it is still controversial whether tangles are a primary causative factor in disease or play a more peripheral role.

Source : Wikipedia