Holding company are those companies which hold the shares of other companies. They are created specifically for the purpose of holding investments, creating layers of holding which may be feasible for tax saving purposes. Many a times the investment held by these companies are valued even more than the market cap of the holding companies but they are held with a view to never sell them.
So, holding companies can be considered as dead investments since the investments are subject to realization if any over very very long period. Therefore chances of holding companies delivering any meaning returns are negligible but they are worth knowing about or having I portfolio if you have a long term view of 5-10 years.