$\text{Computable function}$ is nothing but a function for which there exist an Algorithm.
$\text{Total function}$ is a function which is defined for all possible values of its input.
$\text{Partial functions}$ as the name suggest are the functions which are not defined for all possible values but a proper subset of values of the domain.
For total computable function any value from the domain can be passed in the computable function(total) and it will halt and return a value. Hence the function is decidable.