The term "install base marketing" comes from the industrial era when it referred to the number of machines of a certain manufacturer or brand installed in a particular region.
Let me explain with an example. If you are a software product company for invoicing / billing and your product integrates well with all the CRMs in the market, then your 'install base marketing' approach would target companies using CRM software like Oracle, PeopleSoft, SalesForce and even custom versions.
At times, given the niche, install base marketing could sometimes be the only way of retaining customers and getting new ones. But, are you engaging a dedicated installed base marketing strategy? If not, you could be losing out.