The Strings array property contains the strings in the list, referenced by a zero-based index. Because Strings is the default property for string lists, you can omit the Strings identifier when accessing the list; thus

StringList1.Strings[0] := 'This is the first string.';

StringList1->Strings[0] = "This is the first string.";

is equivalent to

StringList1[0] := 'This is the first string.';

(*StringList1)[0] = "This is the first string.";