In a nutshell:
Glassmorphism is the use of transparency to add depth and a physical property to digital design. It has been widely popularised by Apple who introduced it into their operating systems in 2013 with iOS 7. It has since been used by Microsoft who have used it in their Fluent design system. The design scheme is often used alongside bright colour to draw attention to the blurred effect.
Like other popular visual schemes, such as skeuomorphism, the style is an attempt to draw parallels with real-life physicality. Glassmorphism, however, doesnt mimic objects directly but uses a life-like material to give the illusion of a 3D depth. A refreshing movement away from ultra flat and minimal digital design and UX.