In brief
inset sets the physical top, right, bottom and left offsets of a positioned element. For example, inset: 0 fills the available area with suitable absolute positioning.
Useful to know
inset sets the physical top, right, bottom and left offsets of a positioned element. For example, inset: 0 fills the available area with suitable absolute positioning. [1]
The shorthand changes all four values. When extending an existing component, check that it does not reset an intentional offset on another side. Setting inset alone does not enable positioning.
Sources
For further detail and to check the information: