The term house stock usually refers to a stock

Stock

The capital stock of a business entity represents the original capital paid into or invested in the business by its founders. It serves as a security for the creditors of a business since it cannot be withdrawn to the detriment of the creditors...

 that the management of a brokerage

Stock broker

A stock broker or stockbroker is a regulated professional broker who buys and sells shares and other securities through market makers or Agency Only Firms on behalf of investors...

 firm has instructed all brokers working for it to promote. The brokerage firm or its owners might be receiving an undisclosed profit from the sale of the house stock.

Generally, if you could look at the stock holdings of
all the clients of a boiler room

Boiler room (business)

In business, the term boiler room refers to a busy centre of activity, often selling questionable goods by telephone. It typically refers to a room where salesmen work using unfair, dishonest sales tactics, sometimes selling penny stock or committing outright stock fraud...

 brokerage, almost all clients
would hold the set of house stocks.

A house stock differs from a penny stock

Penny stock

In the United States, penny stocks are common shares of small public companies that trade at less than $1.00. In some countries, similar shares of stock are known as cent stocks.-Concerns for investors:...

in that its price can
be in the normal price range of a large cap stock.