Wikipedia
Positive pressure is a pressure within a system that is greater than the environment that surrounds that system. Consequently, if there is any leak from the positively pressured system it will egress into the surrounding environment.
Use is also made of positive pressure to ensure there is no ingress of the environment into a supposed closed system. A typical example of the use of positive pressure is the location of a habitat in an area where there may exist flammable gases such as found on an oil platform.
This kind of positive pressure also used on operating theaters and in vitro fertilisation (IVF) labs. To maintain positive pressure the room should be sealed to prevent egress of air.
Hospitals may have positive pressure rooms for patients with compromised immune systems. Air will flow out of the room instead of in, so that any airborne microorganisms (e.g., bacteria) that may infect the patient are kept away.
Amphibians use this process to respire, whereby they use positive pressure to inflate their lungs.
This process is important in human and chick development. Positive pressure, created by the closure of anterior and posterior neuropores of the neural tube during neurulation, is a requirement of brain development.