what is mean getters with non-trivial implementations?

A trivial implementation would be one where the getter simply returns the value of a private field

private string _fullname;
public string Fullname
{
    get
    {
        return _fullname;
    }
}

A non trivial example might be a getter that does something non - trivial, such as

public decimal TaxAmount
{
    get
    {
         decimal taxRate = GetTaxRate(Income);
         decimal taxable = Income - TaxFreePay(Income);
         return taxable * taxRate;
    }
}

In other words, a non trivial implementation is just what it says - an implementation that is not trivial.