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Roughly speaking, realism is one of three philosophical positions (the others being nominalism and conceptualism) on the status of universals -- please refer to the universals tag for a brief overview of what universals are.
Realists believe that universals are primarily derived from the "universal features of singular things" meaning that whatever universals are their ontological location (where they are in the world so to speak) is within or before things-in-themselves. Universals, according to realists, primarily inhere within or are situated before things in the world, as opposed to primarily within our minds as concepts or labels. The position of within is referred to as universalia in re (‘universals in the thing’). The position of before as universalia ante rem (‘universals before the thing’). The latter position, where universals are asserted to exist mind independently and thing independently is more or less that of platonism.