High PEEP (Positive End-Expiratory Pressure) refers to elevated levels of pressure maintained in the airways at the end of expiration, typically above 10 cm H2O, to improve oxygenation and lung recruitment in patients with respiratory failure. It is crucial in managing conditions like acute respiratory distress syndrome (ARDS), as it helps prevent alveolar collapse. However, excessive PEEP can lead to complications such as decreased cardiac output due to reduced venous return.