The word 'hyaline' comes from the Greek word hyalos, meaning transparent stone or glass; hyaline tissue or hyaline membrane is a layer of dead cells and other detritus -- specifically, the remains of dead cells, fibrin, and red blood cells -- that collects over the surface of the lungs, coating the alveoli and preventing the exchange oxygen and carbon dioxide. This layer gets its name from its translucent, glassy appearance.
Hyaline tissue forms in cases of acute respiratory distress syndrome (ARDS), which is one of the more dire outcomes of infection with the Covid-19 virus1. It can also form in the lungs of infants, most often in those born prematurely; in this case it is called Hyaline membrane disease or respiratory distress syndrome (RDS). In both ARDS and RDS, hyaline formation is only one symptom.
Hyaline formation is due to damage of both the alveoli epithelial cells and capillary endothelial cells, the remains of which are coated with polymerized fibrin (it is this polymerization that produces the formation of a 'membrane') from the blood plasma. This leads to decreased gas exchange and is a warning sign that the underlying damage is worsening, and may lead to even more severe damage, such as intracapillary thrombosis.
The damage of the surrounding cells and leaking of protein-rich fluids into the alveolar space also interferes with the the surfactant in the alveoli, causing surface tension to make them collapse, and make the lung tissue less compliant. The combination of decreased gas exchange, collapsing alveoli, and stiffer tissue makes breathing very difficult, and is one of the reasons patients often end up on a ventilator.
In the case of premature infants, curing RDS and the associated hyaline membrane is often simply a case of buying them enough time -- usually a few days -- for their body to naturally start producing the pulmonary surfactants that their body was gearing up to produce anyway.
In the case of Covid-19, things are more complicated. In addition to (or instead of) ARDS-type pneumonia, Covid-19 may also result in "non-ARDS pneumonia" (AKA Type 1 or Type L pneumonia), in which the virus attacks the pneumocytes, resulting in serious hypoxemia with no signs of respiratory distress (AKA silent hypoxemia). Because of the significant confusion in causes and high variation in severity of symptoms, it is hard to predict recovery times, and the absence of hyaline tissue may not indicate recovery.
1.AKA Covid-19 associated acute respiratory distress syndrome (CARDS).