noob OOP question about instantiation

Hi there experts!
Please could u help me understanding instantiation and when and how to use it. Firstly, I understand that instantion is creating a new instance of a class for code re-use. For example I have 2 classes, Contractor.cs and DataAccess.cs, with structures as follows:
public class Contractor {

private string _ContractorID = string.Empty;

public Contractor(string ContractorID) {
_ContractorID = ContractorID;
}

public bool IsActive() {
return DataAccess.IsContractorActive(_ContractorID);
}

[...other methods specific to contractor...]
}and:public class DataAccess {

public bool IsContractorActive(string ...