What is domain restrictions and what causes them?

Domain restrictions are values of x which are not acceptable in a function.

They may be arbitrary because we simply want to ignore them. eg y = x^2 between 0 and 4

They may make no sense in the context, eg graph a compounding investment over next 20 years.
Negative values for x make sense here.

They may avoid mathematical impossibilities eg y = (x^2 - 4x) / (x - 4) which must avoid dividing by zero, so x cannot = 4.
The graph will look like the line y = x but have a hole where x= 4