Sharepoint custom fields and SPFieldMultiColumnValue

Sharepoint custom fields and SPFieldMultiColumnValue

I am trying to study custom fields in sharepoint .I have saw the following
code. Can anyone help me to study the SPFieldMultiColumnValue specified
below.
public override object Value
{
get
{
this.EnsureChildControls();
SPFieldMultiColumnValue mcv = new SPFieldMultiColumnValue(2);
mcv[0] = txtDurationId.Text;
mcv[1] = DDLDurationID.SelectedValue;
return mcv;
}
set
{
this.EnsureChildControls();
SPFieldMultiColumnValue mcv =
(SPFieldMultiColumnValue)this.ItemFieldValue;
txtDurationId.Text = mcv[0];
DDLDurationID.SelectedValue = mcv[1];
}
}