Epistemological idealism can mean one of two unrelated positions:
Everything we experience and know is of a mental nature—sense data in philosophical jargon. Although it is sometimes employed to argue in favor of metaphysical idealism, in principle epistemological idealism makes no claim about whether sense data are grounded in reality. As such, it is a container for both indirect realism and idealism.
Knowledge is of a mental nature. That is: ideas, concepts and propositions (and perhaps logic) exist only in the mind and have no extra-mental existence.