basically, domain is all real numbers that cause an not allowed
some things that aren't allowed are dividing by zero and taking the square root of a number less than zero, restrict all values that make thos things hapen
exapmle
a/b, b can not be zero
(3+x)/(4-x)
x cannot be 4 since that would be dividing by zero
another example
√(x-3)
x cannot be less than 3 because that would make a sqrt of a negative
basiccally restrictions are not allowed becasue we don't understnad what happens when we break them