I want to know about "CurrentDB" in detail ?

Dim db As Database
Dim tbl As TableDef
Dim fld As Field
Dim strName As String

?????????? Set db = CurrentDb ?????????

Set tbl = db.TableDefs(tableName)
For Each fld In tbl.Fields
If fld.Name = fieldName Then
FieldExists = True
Exit For
End If
Next
I have no clear idea about the CurrentDb mention above ? pls clear about it.