Any firm which endeavours to extend beyond its own market by invading those of its competitors must incur heavy marketing expenses in order to surmount the barriers by which they are surrounded; but, on the other hand, within its own market and under the protection of its own barrier, each enjoys a privileged position whereby it obtains advantages which--if not in extent, at least in their nature--are equal to those enjoyed by the ordinary monopolist.
Again, this statement is pretty self explanatory and easy to conceptualize. If a firm wants to venture out of their comfort zone into another firm's market in attempt to sell their good, they would have to invest in heavy marketing techniques to get their product known. When entering a market, it is very difficult to market your new product over the familiar competitors good. Therefore, as the rest of the highlighted words suggest, its easier to obtain advantages within your own market, such as owning the familiar commodity.